F. J. Rich

A nearly universal solar wind‐magnetosphere coupling function inferred from 10 magnetospheric state variables

[1] We investigated whether one or a few coupling functions can represent best the interaction between the solar wind and the magnetosphere over a wide variety of magnetospheric activity. Ten variables which characterize the state of the magnetosphere were studied. Five indices from ground-based magnetometers were selected, namely Dst, Kp, AE, AU, and AL, and five from other sources, namely auroral power (Polar UVI), cusp latitude (sin(A c )), b2i (both DMSP), geosynchronous magnetic inclination angle (GOES), and polar cap size (SuperDARN). These indices were correlated with more than 20 candidate solar wind coupling functions. One function, representing the rate magnetic flux is opened at the magnetopause, correlated best with 9 out of 10 indices of magnetospheric activity. This is dΦ Mp / dt = v 4/3 B T 2/3 sin 8/3 (θ c /2), calculated from (rate IMF field lines approach the magnetopause, ∼v)(% of IMF lines which merge, sin 8/3 (θ c /2))(interplanetary field magnitude, B T )(merging line length, ∼(B MP /B T ) 1/3 ). The merging line length is based on flux matching between the solar wind and a dipole field and agrees with a superposed IMF on a vacuum dipole. The IMF clock angle dependence matches the merging rate reported (albeit with limited statistics) at high altitude. The nonlinearities of the magnetospheric response to B T and v are evident when the mean values of indices are plotted, in scatterplots, and in the superior correlations from dΦ MP /dt. Our results show that a wide variety of magnetospheric phenomena can be predicted with reasonable accuracy (r> 0.80 in several cases) ab initio, that is without the time history of the target index, by a single function, estimating the dayside merging rate. Across all state variables studied (including AL, which is hard to predict, and polar cap size, which is hard to measure), dΦ MP /dt accounts for about 57.2% of the variance, compared to 50.9% for E KL and 48.8% for vBs. All data sets included at least thousands of points over many years, up to two solar cycles, with just two parameter fits, and the correlations are thus robust. The sole index which does not correlate best with d ΦMP /dt is Dst, which correlates best (r = 0.87) with p 1/2 dΦ MP /dt. If dΦ MP /dt were credited with this success, its average score would be even higher.

Multiradar observations of the polar tongue of ionization

[1] We present a global view of large-scale ionospheric disturbances during the main phase of a major geomagnetic storm. We find that the low-latitude, auroral, and polar latitude regions are coupled by processes that redistribute thermal plasma throughout the system. For the large geomagnetic storm on 20 November 2003, we examine data from the high-latitude incoherent scatter radars at Millstone Hill, Sondrestrom, and EISCAT Tromso, with SuperDARN HF radar observations of the high-latitude convection pattern and DMSP observations of in situ plasma parameters in the topside ionosphere. We combine these with north polar maps of stormtime plumes of enhanced total electron content (TEC) derived from a network of GPS receivers. The polar tongue of ionization (TOI) is seen to be a continuous stream of dense cold plasma entrained in the global convection pattern. The dayside source of the TOI is the plume of storm enhanced density (SED) transported from low latitudes in the postnoon sector by the subauroral disturbance electric field. Convection carries this material through the dayside cusp and across the polar cap to the nightside where the auroral F region is significantly enhanced by the SED material. The three incoherent scatter radars provided full altitude profiles of plasma density, temperatures, and vertical velocity as the TOI plume crossed their different positions, under the cusp, in the center of the polar cap, and at the midnight oval/polar cap boundary. Greatly elevated F peak density (>1.5E12 m 3 ) and low electron and ion temperatures (2500 K at the F peak altitude) characterize the SED/TOI plasma observed at all points along its high-latitude trajectory. For this event, SED/TOI F region TEC (150–1000 km) was 50 TECu both in the cusp and in the center of the polar cap. Large, upward directed fluxes of O+ (>1.E14 m 2 s 1 ) were observed in the topside ionosphere

Ionospheric effects of major magnetic storms during the International Space Weather Period of September and October 1999: GPS observations, VHF/UHF scintillations, and in situ density structures at middle and equatorial latitudes

In this paper we present a study of the ionospheric effects of a halo coronal mass ejection (CME) initiated on the Sun on September 20, 1999, and causing the largest magnetic storm during this month on September 22–23, 1999, with the hourly Dst index being −167 nT at ∼2400 UT on September 22. The recurrent CME on October 18 caused an even larger magnetic storm on October 22, 1999, with Dst of −231 nT at ∼0700 UT. The ionospheric effects of these two major magnetic storms are studied through their effects on a prototype of a Global Positioning System (GPS)-based navigation system called Wide Area Augmentation System (WAAS) being developed by the Federal Aviation Administration for use in the continental United States and their impact on global VHF/UHF communication systems. It is shown that the penetration of transient magnetospheric electric fields equatorward of the shielding region at midlatitudes, which have been well-correlated in the past with rapid changes in the well-known Dst index (or through its recently available high resolution 1-min counterpart the SYM-H index), can cause large increases of total electron content (TEC), TEC fluctuations, and saturated 250-MHz scintillation, and these, in turn, may have significant impacts on WAAS. The local time of Dst changes (and not just Dst magnitude) was found to be very important for WAAS, since the largest effects on TEC are seen near dusk. The prompt penetration of these magnetospheric electric fields all the way to the magnetic equator causes augmentation or inhibition of equatorial spread F. The global ionospheric response to these storms has been obtained from ground-based TEC observations with a GPS network and space-based in situ density and electric field measurements using the Republic of China Satellite-1 (ROCSAT-I) and several Defense Meteorological Satellite Program satellites. These prompt penetration electric fields cause VHF/UHF scintillations and GPS TEC variations at low latitudes in the specific longitude sector for which the early evening period corresponds to the time of rapid Dst variations and maximum Dst phase. The effects of the delayed ionospheric disturbance dynamo and those of decreased magnetospheric convection on postmidnight irregularity generation are shown to be confined to a part of the same longitude range that actively responded to the prompt penetration of electric fields in the early evening sector.

Multisatellite observations of rapid subauroral ion drifts (SAID)

We present the first conjugate observations of subauroral ion drifts (SAID) in the magnetosphere (∼9000 km altitude) and ionosphere and coincident measurements by four ionospheric satellites. The parameters measured include ion drifts, energetic precipitating electrons and ions, and the magnetic field perturbations associated with field-aligned currents. Observations indicate that SAID are very coherent features that occur simultaneously over a large magnetic local time (MLT) range, from at least 1600 to 2400 MLT. The equatorward extent of SAID, the ion precipitation, and the region 2 field-aligned currents (FAC) flowing into the ionosphere are all shown to be coincident at all MLT locations where SAID are observed. They also appear to be closely related to the conductivity distribution in the subauroral ionosphere and the midlatitude trough. This is interpreted as an indication that their latitudinal distribution is a consequence of the subauroral conductivity structure and the movement of the plasma sheet ion and electron boundaries. Conjugate measurements at diverse altitudes when mapped along field lines are nearly identical, indicating the absence of significant field-aligned potential drops. Temporally separated SAID measurements in similar MLT regions show a reduction with time in the field-aligned current densities with little reduction in the potential drop across the SAID. We interpret the results as an indication that the magnetosphere acts as a current generator in which large FAC are initially required to support the electric field gradient in a SAID event. Subsequent evolution in the E and F regions produces large conductivity gradients that are in the right sense to remove the intense FAC requirement but maintain the large subauroral electric fields. The reported potential drops in the subauroral region can be a significant fraction of the total, up to 60 kV or more, and must be taken into account when deriving any magnetospheric convection pattern.

Polar cap index (PC) as a proxy for ionospheric electric field in the near‐pole region

The ion drift measurements made by a number of DMSP satellites during some intervals in 1991, 1997, and 1998 are utilized for estimation of the ionospheric electric fields over the near-pole region; these estimates are then compared with the Polar Cap (PC) magnetic activity index obtained from ground geomagnetic observations at Qaanaaq (former Thule, Greenland) and Vostok (Antarc- tica). The analysis shows that the polar cap electric field is primarily controlled by variations in the near-Earths inter- planetary electric field. The relationship between the polar cap ionospheric electric field and the PC-index can be ap- proximated by a quadratic polynomial. The polar cap iono- spheric electric field tends to saturate at the asymptote of -45-50 mV/m when the PC index reaches large positive values (PC > 10); the residual electric field (for near-zero interplanetary electric field applied to the Earths magneto- sphere) is-12 mV/m. It is concluded that the PC-index can serve as a proxy of the ionospheric electric fields in the near-pole region.

Analysis of the ionospheric cross polar cap potential drop using DMSP data during the National Space Weather Program study period

During the National Space Weather Program (NSWP) event study period of November 2–11, 1993 there were three operational DMSP meteorological satellites (F8, F10, and F11) in orbit, each carrying the Special Sensor for Ions, Electrons, and Scintillation (SSIES) plasma instrument package. Ion flow data from these instruments are used to determine the electrostatic potential drop across both the northern and southern polar caps. The magnitude and distribution of the potential are used to characterize the convection patterns present in the polar ionospheres. The results from all three satellites ate presented to show an overall observational history of the potential drop and the convection pattern during the study period. These observational parameters provide crucial inputs and checks to several ionospheric and magnetospheric models being used in this study. Evidence is presented of an unambiguous difference in the cross polar cap potential drop between the two hemispheres for an extended period of time.

Redistribution of the Stormtime Ionosphere and the Formation of a Plasmaspheric Bulge

Plasmasphere drainage plumes resulting from the erosion of the plasmasphere boundary layer by disturbance electric fields have been identified from both ground and space. Here we describe a localized enhancement of total electron content (TEC) seen at the base of the erosion plume, on field lines mapping into the outer plasmasphere. Observations suggest that this enhanced TEC results from a poleward redistribution of post-noon sector low latitude ionospheric plasma during the early stages of a strong geomagnetic disturbance. Ground based and low- altitude observations with GPS TEC, incoherent scatter radar, and DMSP in situ observations provide details and a temporal history of the evolution of such events. Seen from space by IMAGE EUV, the region of enhanced TEC appears as a pronounced brightening in the inner plasmasphere. IMAGE FUV provides complementary images at lower altitude of this inner-plasmasphere feature, showing that it is associated with localized enhancement in the vicinity of the equatorial anomaly peak. These effects are especially pronounced over the Americas, and we suggest that this results from a strengthening of the equatorial ion fountain due to undershielded (penetrating) electric fields in the vicinity of the South Atlantic magnetic anomaly. The enhanced low-latitude features, seen both from the ground and from space, corotate with the Earth once they are formed. The high-TEC plasma in these regions contributes to the intensity of the erosion plumes arising in the American sector during strong disturbance events.

Comparison of ion densities measured in the topside ionosphere at low latitudes and midlatitudes with calculations of ionospheric models over a full solar cycle

Measurements of total ion density at 840 km altitude at all latitudes in four local time sectors are available from 1987 to the present from spacecraft of the Defense Meteorological Satellite Program (DMSP). Thus all phases of the past solar cycle are represented in the data set. We present comparisons of the measurements with values obtained from climatology models of the ionosphere. The models examined are the International Reference Ionosphere (IRI) model, the Parameterized Ionospheric Model (PIM) and the Reilley-ICED-Bent- Gallagher (RIBG) model. We show that all of the models reproduce some of the features of the observed topside ionosphere, but none of the models match all of the observations.

Low energy auroral electron and ion hemispheric power after NOAA and DMSP intersatellite adjustments

Twenty-eight years of low energy auroral electron (Hpe) or total (Hpt, electron plus ion) hemispheric power estimates from 11 NOAA (< 20 keV) and 11 DMSP (< 30 keV) satellites are adjusted to produce consistent electron hemispheric power (Hpe) estimates for the south and north hemispheres. Most satellites sample the nighttime aurora in the southern hemisphere, so the hemispheric power estimates are more consistent (i.e. better) in the southern hemisphere. The low energy (<20 keV) auroral ion hemispheric power (Hpi) estimates are found from the SEM-2 NOAA instruments beginning in 1998. The ion hemispheric power is the difference of the estimated total hemispheric power minus the estimated electron hemispheric power (i.e. Hpi = Hpt – Hpe). The ion hemispheric power estimates are less reliable than the corresponding electron hemispheric power estimates, especially in the northern hemisphere. Corrections are made for sunlight contamination, data dropouts over the auroral oval, the degradation of sensors over time, high spurious count rates, and increased noise at the end of a satellite lifetime. Adjustments are made such that the ratios of Hpe or Hpi estimates from one satellite to another are 1.00+/-0.05. Intersatellite cross-correlation coefficients exceed 0.70 most of the time and are 0.08 better (about 10% better) in the southern hemisphere than in the northern hemisphere. In addition, adjustments are made so the ratio of the south to north electron or ion hemispheric power from each satellite is approximately equal to 1.0 over a satellite lifetime. Since DMSP satellites undergo both pre-flight and post-flight calibration adjustments, the chosen Hpe baseline is near the median of the original DMSP Hpe estimates, where the original DMSP-F13 Hpe estimates are closest to the final intersatellite adjusted Hpe estimates. The Hpi baseline is chosen such that the adjusted average composite hourly SEM-2 southern hemisphere Hpi (<20 keV) for all seasons for the three Kp levels between 1and 3+ are 77+/-1% of the DMSP Hpi (<30 keV) estimates for the same Kp levels by Hardy et al. [1989]. This baseline level is chosen because DMSP-F13 and F15 ions < 20.62 keV carry 82% to 72% (77+/-5%) of the along-track ion energy flux for ions < 30.18 keV in southern hemisphere (SH) orbits for 123 days in 2001 with Kp between 1and 3+. Hourly electron and ion hemispheric power composites are created with the number of estimates, and the maximum, median, mean and standard deviation of the estimates during each hour. Daily medians and averages for each hemisphere are made from these hourly median and average composites, respectively. The southern hemisphere medians of the daily medians of the electron and ion hemispheric power are 12.2 GW and 1.10 GW, while the averages of the daily averages are 17.6 GW and 1.47 GW, respectively. Winter values of the electron hemispheric power exceed the summer values by up to 10% in solar minimum and up to 50% in solar maximum. Summer low energy (<20 keV) ion hemispheric power estimates are 20 to 50% larger than in winter. Hemispheric power estimates from all individual satellites are revised using the various adjustments to create a new data set of intersatellite adjusted values of Hpe for all satellites and Hpi and Hpt for NOAA SEM-2 satellites. The hourly and daily electron and ion composites and the adjusted individual satellite estimates of the electron and ion

Observed and predicted potential distributions during the October 1995 magnetic cloud passage

Ion drift measurements from DMSP F13 provide the first opportunity to study the evolution of high latitude potentials in both hemispheres as a magnetic cloud passed Earth. Early in the event with IMF B Z strongly southward, the polar cap expanded and the potential measured across it reached 174 kV, then decreased as the IMF rotated toward B Y < 0. During the initial northward IMF phase convection in the polar cap was dominated by large lobe cells, clockwise in the southern and counterclockwise in the northern hemisphere. With purely northward IMF, convection patterns had four cells and were irregular on the day and night sides of the magnetic dawn-dusk meridian. Potential distributions measured with a southward or westward IMF agree with predictions of the Weimer [1996] model, suggesting that some global MHD simulations overestimate dayside merging rates.