We have explored the use of COSMIC data to provide valuable scientific
information on the ionospheric impacts of energetic particle precipitation
during geomagnetic storms. Ionospheric electron density in the E region, and
hence ionospheric conductivity, is significantly altered by precipitating
particles from the magnetosphere. This has global impacts on the
thermosphere–ionosphere because of the important role of conductivity on
high-latitude Joule heating. Two high-speed stream (HSS) and two coronal
mass ejection (CME) storms are examined with the COSMIC data. We find clear
correlation between geomagnetic activity and electron density retrievals
from COSMIC. At nighttime local times, the number of profiles with maximum
electron densities in the E layer (below 200 km altitude) is well correlated
with geomagnetic activity. We interpret this to mean that electron density
increases due to precipitation are captured by the COSMIC profiles. These
“E-layer-dominant ionosphere” (ELDI) profiles have geomagnetic latitudes
that are consistent with climatological models of the auroral location. For
the two HSS storms that occurred in May of 2011 and 2012, a strong
hemispheric asymmetry is observed, with nearly all the ELDI profiles found
in the Southern, less sunlit, Hemisphere. Stronger aurora and precipitation
have been observed before in winter hemispheres, but the degree of asymmetry
deserves further study. For the two CME storms, occurring in July and
November of 2012, large increases in the number of ELDI profiles are found
starting in the storm's main phase but continuing for several days into the
recovery phase. Analysis of the COSMIC profiles was extended to all local
times for the July 2012 CME storm by relaxing the ELDI criterion and instead
visually inspecting all profiles above 50
The high-latitude ionosphere, encompassing the auroral and polar cap regions, is an area of important processes related to magnetosphere–ionosphere coupling that has global impacts on the thermosphere–ionosphere system. New scientific results have come from coordinated use of ground and space observations. The high latitudes are reasonably well instrumented with ground-based data, including radars, imagers and GPS receivers (total electron content) that cover wide geographic areas. Operational satellite missions are providing data on a continuous basis from high latitudes.
Despite these resources, critical quantities are only observed indirectly or
sparsely, such as the changes in high-latitude conductivity resulting from
energetic particle precipitation (EPP). Ionospheric electron density, and hence
conductivity, is significantly altered by precipitating particles from the
magnetosphere, which has global impacts because of the important role of
conductivity on high-latitude Joule heating (Cole, 1962; Wiltberger et al.,
2004). The community has devoted significant resources to developing
climatological models that provide quantitative information on auroral zone
precipitation and the resultant conductivities (Robinson et al., 1987;
Fuller-Rowell and Evans, 1987; Zhang and Paxton, 2008). A lack of
observations has hampered more detailed understanding of these processes,
because it requires simultaneous observations in the lower ionosphere
(
The Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC)
is a constellation of six orbiting satellites with onboard GPS
receivers that acquire total electron content data in limb viewing
geometries as the GPS satellites occult behind Earth from the perspective of
the low-Earth orbiters. Using the assumption of local spherical
symmetry near the ray path tangent point, profiles of electron
density versus altitude are retrieved. Although local spherical symmetry is
not realistic at high latitudes, COSMIC data can be an extremely sensitive
monitor of narrow (
In the next section we describe the COSMIC observations and how they are used to detect EPP signatures in the upper atmosphere. We then present results for four geomagnetic storms, originating from both high-speed streams (HSSs), and coronal mass ejections (CMEs). The results are then discussed and a conclusion and suggestions for further work follow.
Each COSMIC satellite carries a dual-frequency GPS receiver and two antennas that acquire and track GPS satellites that are in front of and behind the satellite (Fong et al., 2011; Rocken et al., 2000). The total electron content (TEC) between satellite and transmitter is derived by differencing the carrier phase and pseudorange delays measured at the L1 and L2 frequencies (Schreiner et al., 1999; Hajj and Romans, 1998). Subsequent processing requires the calibration of hardware differential delays that bias the TEC (Stephens et al., 2011). Finally, an Abel integral is applied to the TEC data to yield approximate vertical profiles of electron density versus altitude (Hajj and Romans, 1998; Schreiner et al., 1999). Data are obtained at a cadence of 1 s.
Previous studies of COSMIC electron density profiles have revealed good
retrieval accuracy (to within
We focus on the presence of E region layers in the electron density profiles
with peak densities below 200 km altitude. At geomagnetic latitudes above 60
Four representative COSMIC electron density profiles from high latitude. Top: no electron density enhancements; bottom: electron density enhancements, assumed due to energetic particle precipitation, indicated by arrows.
Solar wind parameters from the OMNI data set for the four events
studied is this paper.
Continued.
Continued.
Continued.
In Fig. 2a–d we have plotted interplanetary parameters for the
four geomagnetically active intervals studied in this paper, obtained from
the OMNI data set at CDAWeb (
Interplanetary parameters for the two fast CME
events are shown in Fig. 2c and d (July and November 2012,
respectively). These are more intense than the CIR-driven geomagnetic
storms. These interplanetary CME (ICME) events are characterized by a shock,
a sheath and then a magnetic cloud (MC). The shock is characterized by a sudden
increase in solar wind velocity, density and temperature and increased
magnetic field magnitude. The sheath lies between the shock and the MC, which is a remnant of the CME proper. In both cases, the
southward orientation of the magnetic fields within the MC leads to
significant magnetic storms (Dst <
The high-speed stream storms had apparent longer durations than the ICME storms did. This is because the southward component of the Alfven waves in the high-speed stream proper continue to pump energy into the magnetosphere well after the CIR has passed (Tsurutani and Gonzalez, 1987).
Previous authors have reported on the need to apply QC to
the COSMIC electron density profile data (Sheng et al., 2012). This QC is
meant to remove gross outliers, not to correct for retrieval errors. It
affected over 20 % of the profiles used in the Sheng et al. (2012) study
(C. Sheng, private communication, 2012). Analysis at JPL suggests that recent
versions of processing at the COSMIC Data Analysis and Archive Center
(CDAAC,
Altitude of peak electron density for the four storm periods studied in this paper. MLT range is 21:00–05:00. All profiles within the MLT range are plotted for Northern (red) and Southern (blue) hemispheres.
E layer dominant electron density profile locations versus magnetic latitude and MLT for selected days in the four storm periods. MLT range is 21:00–05:00. Green circles are Northern Hemisphere. Cyan circles are Southern Hemisphere. A line connects each circle to the climatological equatorward boundary of the auroral oval, indicated as a black triangle. The boundary is determined based on the IRI2012 model computation (Bilitza et al., 2014; Zhang and Paxton, 2008).
To investigate the signature of EPP on COSMIC electron density profiles, we
analyzed the altitude of the electron density peak value with altitude. When
the peak occurs below
The distribution of the height of electron density maximum
Locations of COSMIC electron density profiles for the July 2012 storm period in MLT/geomagnetic latitude coordinates for the Northern Hemisphere. Green circles indicate the presence of a E region density layer, open red circles indicate no layer present, and blue circles are ambiguous. The climatological boundary from IRI2012 is indicated as in Fig. 4.
Figure 4 shows the locations of the profiles in Fig. 3 for which the
electron density maximum occurs below 200 km, i.e., the profiles showing
significant precipitation. Two days for each storm are shown: the day when
the maximum number of precipitation events occurs and a day in the
following recovery period. Magnetic local time is restricted to 21:00–05:00 MLT,
as in Fig. 3. Northern Hemisphere cases are in green; Southern Hemisphere is cyan. Also shown is the location of the climatological
equatorward boundary of the auroral oval corresponding to each profile as
determined by the model of Zhang and Paxton (2008), which has been
incorporated into the 2012 version of the International Reference Ionosphere
(Bilitza et al., 2014). This boundary is determined by auroral images from
the SUSSI series of satellites as a function of the
The width in geomagnetic latitude of the region where precipitation occurs
is generally in the range 60–75
The situation is somewhat more complex for one of the CME-driven events studied here. For the July 2012 event, there is a systematic underestimation by the model of the precipitation's equatorward boundary during the storm commencement on 14 July 2012, even though the equatorward boundary moves to lower latitudes as the storm intensifies (see Fig. 5). Lower-latitude precipitation also occurs during the storm's recovery phase, as discussed in more detail in Sect. 4.
A more detailed analysis is presented of the July 2012 CME event. We wish to
remove the restriction to nighttime local times in order to consider all local times
for latitudes above 50
Rather than develop an automated algorithm, which was not straightforward,
we visually inspected profiles to discern the presence of enhanced electron
density layers at E region altitudes. All profiles above 50
The results for the period 14–20 July 2012 are shown in Fig. 5. A marker is placed in a geomagnetic latitude–MLT coordinate system for each electron density profile. Marker color indicates the presence or absence of scintillation. Open red circles indicate profiles that show no evidence of an enhanced E layer density. Green circles indicate clear evidence of a density enhancement layer below 200 km altitude. Small blue circles are ambiguous, for example: data are not available below 150 km altitudes, or the profile is irregular. Also shown, as black triangles, are the climatological equatorward boundaries of precipitation from the Zhang/Paxton model used in IRI 2012.
The fraction of profiles displaying precipitation increases significantly during the storm main phase on 15 July. This continues through 17 July, whereas on 18 July the number of profiles showing E layer density enhancement reduces significantly and remains low. On the 3 most active days (15, 16 and 17 July), there is little evidence of a preferred local time for precipitation signatures. Precipitation patterns and their significance are further discussed in Sect. 4.
The science question addressed in this paper is whether COSMIC data can improve knowledge of how conductances change during geomagnetic storms. Such improved knowledge could have a significant impact on the accuracy and utility of global coupled models of the magnetosphere–thermosphere–ionosphere. Since Pedersen and Hall conductances are calculated based on electron density (Kelley, 2009), data that bear directly on conductances have been limited. Global circulation models of the thermosphere–ionosphere have used a variety of climatological approaches to estimate conductances. A widely used method of obtaining conductance patterns is based on the work of Fuller-Rowell and Evans (1987). Satellite measurements of precipitation, e.g., by the NOAA satellites (Asikainen and Mursula, 2013), are the input for selecting different conductance patterns. By the nature of this and similar approaches, conductances are approximate and could be refined considerably with direct measurements of electron density. COSMIC provides this possibility.
The relationship between particle properties measured by satellites and conductance patterns in the ionosphere involves multiple assumptions. Robinson et al. (1987) used the assumption of Maxwellian energy distributions of precipitating electrons to derive a relationship between height-integrated conductivity and measured precipitation properties. They found that this relationship is strongly dependent on the spectrum of electron energies. Recent work by Sheng et al. (2014) has revealed significant differences between measured conductance estimates using COSMIC and those estimated using TIE-GCM, a community general circulation model of the coupled thermosphere–ionosphere. Their focus was the ratio of E to F layer conductances (height-integrated within each layer), a key ratio that determines how the thermosphere temperature and composition is modified by electrodynamics. This study encompassed several seasons over the years 2008–2011 and was focused on geomagnetically quiet periods. It is reasonable to suggest that disturbed periods will show similar or larger discrepancies between COSMIC measurements and climatologies.
Other indirect methods of obtaining conductances have relied on auroral imagery (Aksnes et al., 2005). However, auroral images are challenged during daytime local times due to interference from dayglow. Multiple assumptions regarding atmospheric properties and how UV emission is related to electron energy spectra are required. COSMIC conductivity measurements have the potential to be more direct and add significantly to our understanding of conductance changes during geomagnetic storms.
Figure 3 shows the altitudes of the peak electron density on a daily basis for the four storm intervals for nighttime MLT. Increased precipitation, whose signature is peak electron density altitudes near 200 km versus near 350 km, during the magnetic storm periods (see Fig. 2) is clear in these plots. Whereas the ICME storms show a greater fraction of profiles with precipitation signatures during the most intense days, the numbers of profiles showing precipitation tends to be more consistent over time for the HSS storms. This may be associated with continuous driving of the magnetosphere, associated with HILDCAAs, for these HSS storms. Recent research suggests that HILDCAAs are always associated with relativistic precipitation (Hajra et al., 2014). Our limited study suggests that it is worth pursuing the hypothesis that long-duration auroral geomagnetic activity brings a steady stream of precipitating particles into the high-latitude E region.
A notable feature of the two HSS storms is the strong asymmetry of response between the Northern and Southern hemispheres. Both storms occurred in the northern spring season. The 2011 storm starts with approximately equal numbers of ELDI profiles in both hemispheres but ends on 4 May 2011 with nearly all ELDI profiles in the Southern Hemisphere. The May 2012 storm shows several days with predominantly southern hemispheric ELDI profiles. Since the formation of ELDI depends on the magnitude of the F region peak density, solar zenith angle considerations may play a role in the observed hemispheric asymmetry: ELDI requires larger E region density enhancements in the Northern, more sunlit, Hemisphere. However, solar illumination considerations cannot fully explain the observed asymmetry since it occurs consistently at a wide range of geomagnetic latitudes, but the solar illumination hemispheric differences decrease with latitude. Also, the asymmetry varies throughout the storm whereas solar illumination will remain constant over the storm period.
Two causes of auroral hemispheric asymmetry are generally cited in the
literature: season and orientation of the interplanetary magnetic field (see
Newell et al., 2010, and Østgaard and Laundal, 2012, respectively). Aurora
tend to be more intense in the less sunlit hemisphere due to the lower
conductivity that increases certain aspects of magnetosphere–ionosphere
coupling. That would tend to favor what is observed, which is more ELDI
profiles in the Southern Hemisphere. Conversely, a cause of the asymmetry
due to the interplanetary magnetic field (IMF) is harder to justify due to the very large fluctuations in the
IMF for high-speed streams. In fact, for the May 2012 event the orientation
of IMF, despite the fluctuations, changes between 13 and 14 May. For
both days (Fig. 2b),
The MLT/latitude locations of the ELDI profiles are shown in Fig. 4 for
selected days. The range of MLT is 21:00–05:00, as with Fig. 3. Northern
Hemisphere profiles are in green, and Southern Hemisphere profiles are in
cyan. The locations of the climatological equatorward boundary are shown as
black triangles, connected by a line to the profile location. The
climatology reinforces our interpretation of the ELDI profiles as caused by
precipitation, since in most cases the profiles lie poleward or near the
climatological lower-latitude auroral boundary. An exception appears to be
for CME storm starting on 14 July 2012, where a significant number of the
profiles are equatorward of the boundary by about 5–10
The candidate nighttime precipitation signatures follow the geomagnetic storm evolution, with the signatures increasing and decreasing on a daily basis in concert with the geomagnetic storm intensity. Since the nighttime precipitation was considered only if the E layer density exceeds density in the F layer, there are likely additional precipitation signatures in the data that are not shown in Figs. 3 and 4. The time history of profiles that display E region electron density maxima, and their locations relative to the estimated equatorward boundary of the auroral oval, are consistent with the profiles being reliable indicators of energetic electron precipitation. Our results suggest that COSMIC electron density profiles can provide valuable information on the distribution of conductance changes due to precipitation during geomagnetic storms, thus contributing to a long-standing issue in magnetosphere–ionosphere coupling. The magnitude of the conductance changes can be estimated also, particularly for profiles with more prominent E layer density increases such as the ELDI profiles.
Figure 5 shows a more detailed analysis of the July 2012 CME storm in the Northern Hemisphere. For this case, we consider all magnetic local times and
latitudes down to 50
Auroral storm activity began on 14 July (Fig. 2c), although the magnetic storm (Dst, ring current) had not yet entered the main phase. The number of precipitation events has clearly begun to increase on the 14th (Fig. 5) relative to the 2 previous quiet days. A predominance of the precipitation cases is on the nightside.
Significantly enhanced precipitation is observed on days 15–17 July, which
encompasses the main phase of the storm and the start of the prolonged
recovery phase (note that a smaller new storm is initiated on 17 July
by the
A notable feature of the storm-time precipitation is its nearly uniform distribution in magnetic local time. Further analysis is required to determine how the impact of the precipitation, as measured by the increased E layer electron densities, is distributed in MLT. Determining impact is complicated by the fact that COSMIC electron density profiles in the E region are often not accurate to better than 20 %, and errors can exceed 100 %, due to horizontal plasma gradients affecting the Abel retrievals (Yue et al., 2010). Nevertheless it is clear that precipitation signatures are not confined to the nightside.
The uncertainty of E layer electron densities affects how well ionospheric conductivities can be calculated using radio occultation data, which depend linearly on the electron density (Sheng et al., 2014). The importance of ionospheric conductivities for understanding magnetosphere–ionosphere coupling suggests that further work should be focused on reducing electron density uncertainty for high-latitude profiles.
Green circles are auroral boundary locations (decimated) from the SSUSI sensor onboard DMSP satellite F18. The climatological boundary from IRI2012 is indicated as in Fig. 4.
The characteristics of E region errors in radio occultation are not likely to produce false positive identifications of precipitation using the criteria we are applying. Due to the nature of the retrieval process (Hysell, 2007; Yue et al., 2010; Hajj and Romans, 1998), retrieval errors will not introduce a single electron density enhancement layer of the kind we are using to identify precipitation. The background E region electron density, defined as the smoothly decreasing component, could certainly be in error. If overestimated, errors in the background might mask a small precipitation enhancement layer. As stated in Sect. 3, multiple precipitation layers were excluded from identification. Thus, we are not subject to false positives due to noisy retrievals as described by Hysell (2007). When excess noise was visible in the E region, we denoted this as an “ambiguous” case. The number of ambiguous cases is not overwhelming our analysis (Fig. 5).
Another notable feature of the storm-time precipitation is its latitude relative to the climatological equatorward boundary. An increase of precipitation signatures equatorward of the boundary is seen on storm days 16–17 July relative to 15 July. This may indicate that COSMIC data are capable of contributing to refined climatologies when combined with existing data sources such as SSUSI and GUVI, from which the climatology is derived. On average, the climatological auroral boundary is most equatorward on 15 July, when the main storm phase occurs, and then retreats somewhat poleward during the recovery phase on 16–17 July. However, the data show a different pattern of largely maintaining the equatorward extent of the precipitation during the recovery phase compared to the main phase. Persistently increased plasma at low L shells during the recovery is not surprising and could be the source of the lower-latitude precipitation. It is not clear why the climatology does not capture this effect.
These results are compared with the auroral equatorward boundary product
from the SSUSI ultraviolet sensor of the Defense Meteorological Satellite
Program (DMSP) satellite F18 (Paxton et al., 2002;
The SSUSI data confirm that, during quiet time, the high-latitude COSMIC
E region density layers often fall outside the auroral zone when they
appear. During disturbed days starting on 14 July, the equatorward expansion
of the auroral oval is largely consistent between COSMIC and SSUSI data
until 17 July and after, during the storm's recovery phase (and after a
17 July substorm captured by the Dst index). On 17 July in particular, the
SSUSI nightside auroral boundary has retreated to latitudes generally
poleward of 65
Comparisons were made with online runs of the Ovation auroral model to assess how COSMIC data sample the poleward boundary of the oval. Ovation is a climatological model of the location of the auroral oval and the intensity of auroral precipitation (Newell et al., 2002). It also accepts real-time data, primarily from the DMSP series of satellites (Hardy et al., 1985). Although Ovation output was not available continuously for the July 2012 storm period, output was available several times per day. We used the latitude extent of the aurora from Ovation, and its temporal changes, to gain further insight into the COSMIC data.
Throughout the 15–17 July storm period, the Ovation oval is mostly confined
to latitudes 60–75
The poleward boundary of Ovation moves somewhat more northward on the
16th compared to the 15th. COSMIC data seem to show agreement
with this qualitative feature in that there are a higher fraction of
profiles with precipitation signature poleward of 75
The equatorward boundary of Ovation during the recovery phase on 16 July moves poleward compared to 15 July, as would generally be expected during the recovery phase. However, the COSMIC data tend to have a higher fraction of lower-latitude precipitation events on the 16th, as noted earlier. Thus, there is general disagreement between the COSMIC data and the two climatologies: COSMIC suggests increased precipitation at lower latitudes during this part of the recovery phase, whereas the models suggest poleward retreat of the lower-latitude boundary.
The recovery phase is interrupted on 17 July by a significant
southward turning of IMF
Ovation runs for 17 July extend the poleward extent of the oval to
near 80
On 20 July, five profiles out of 15 are at or below 60
Ionospheric conductance changes during storms remain an important topic due to its influence on the thermosphere–ionosphere both regionally and globally. Global circulation models of the thermosphere–ionosphere suggest significant dependencies on precipitation patterns and the details of high-latitude energy deposition. We have used COSMIC electron density profiles to analyze the presence of enhanced electron density layers in the E region and determine whether these layers are consistent with ionization from energetic particle precipitation from the magnetosphere during geomagnetic disturbances. We have examined two types of ionization signature: nighttime signatures where the maximum electron density occurs at altitudes below the ionospheric F layer and evidence of individual precipitation layers at altitudes below the F region.
We considered two storms (in 2011 and 2012) that began when CIRs in the solar wind were formed by high-speed stream interaction with the slow solar wind. Notable features of nighttime E-region-dominant ionosphere profiles for these two events are increased precipitation that continued for several (5–6) days as the AE index remained elevated; and a strong hemispheric asymmetry, favoring more precipitation in the Southern Hemisphere particularly several days following storm initiation.
We considered two CME-originating storms, both in 2012, which showed larger numbers of daily E-region-dominant profiles on the peak days than the CIR-HSS storms. For the July 2012 storm, the number of E-region-dominant profiles was large also during the storm recovery phase. A more detailed study of the July 2012 CME storm relaxed the requirement of peak density in the E region to consider all electron density profiles with evidence of an enhanced E region layer due to precipitation. Precipitation signatures were found nearly uniformly distributed in local time for the 3 storm days 15–17 July, which includes the main phase and 2 days of recovery phase. On 16–17 July, precipitation was often observed equatorward of the climatological auroral boundary. On 17 July in particular, precipitation from COSMIC was often equatorward of the auroral boundary derived from the F18 DMSP satellite SSUSI sensor. Very-high-latitude precipitation was observed also, although at a lower incidence rate. This bears further study in light of recent research by Huang et al. (2014) that polar cap energy deposition can be significant during storms.
Despite concerns of E region electron density accuracy from COSMIC retrievals, these results suggest that, if care is exercised, COSMIC data from the E region can be used to learn about conductivity changes during geomagnetic storms in the auroral and polar cap regions. COSMIC profiles with enhanced electron density layers are consistent with expected characteristics of where and when precipitation occurs. The magnitude of the E layer ionization above background is probably difficult to determine using COSMIC Abel-retrieved profiles alone. Despite this limitation, the broad high-latitude coverage of COSMIC profiles is likely to be an excellent resource for characterizing the high-latitude consequences of MI coupling.
This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. We acknowledge the support of the Living With a Star Program, within the Heliophysics Division of NASA's Science Mission Directorate. We gratefully acknowledge the provision of Formosat-3/COSMIC electron density profile data from the COSMIC Data Analysis and Archive Center and SSUSI data from the Johns Hopkins University Applied Physics Laboratory. We acknowledge use of NASA/Goddard Space Flight Center's Space Physics Data Facility's OMNIWeb service and OMNI data. Edited by: J. Y. Liu