Electron precipitation characteristics during isolated, compound and multi-night substorm events (original) (raw)
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Journal of Geophysical Research, 2012
Using ground-based subionospheric radio wave propagation data from two very low frequency (VLF) receiver sites, riometer absorption data, and THEMIS satellite observations, we examine in detail energetic electron precipitation (EEP) characteristics associated with two substorm precipitation events that occurred on 28 May 2010. In an advance on the analysis undertaken by Clilverd et al. (2008), we use phase observations of VLF radio wave signals to describe substorm-driven EEP characteristics more accurately than before. Using a >30 keV electron precipitation flux of 5.6 Â 10 7 el. cm À2 sr À1 s À1 and a spectral gradient consistent with that observed by THEMIS, it was possible to accurately reproduce the peak observed riometer absorption at Macquarie Island (L = 5.4) and the associated NWC radio wave phase change observed at Casey, Antarctica, during the second, larger substorm. The flux levels were near to 80% of the peak fluxes observed in a similar substorm as studied by Clilverd et al. (2008). During the initial stages of the second substorm, a latitude region of 5 < L < 9 was affected by electron precipitation. Both substorms showed expansion of the precipitation region to 4 < L < 12 more than 30 min after the injection. While both substorms occurred at similar local times, with electron precipitation injections into approximately the same geographical region, the second expanded in an eastward longitude more slowly, suggesting the involvement of lower-energy electron precipitation. Each substorm region expanded westward at a rate slower than that exhibited eastward. This study shows that it is possible to successfully combine these multi-instrument observations to investigate the characteristics of substorms.
Observations in the vicinity of substorm onset: Implications for the substorm process
1995
Multi-instrument data sets from the ground and satellites at both low and high altitude have provided new results concerning substorm onset and its source region in the magnetosphere. Twenty-six out of 37 substorm onset events showed evidence of azimuthally spaced auroral forms (AAFs) prior to the explosive poleward motion associated with optical substorm onset. The azimuthal wavelengths associated with these onsets were found to range between 132 and 583 km with a mean value of 307 _+ 115 km. The occurrence rate increased with decreasing wavelength down to a cutoff wavelength near 130 km. AAFs can span 8 hours of local time prior to onset and generally propagate eastward in the morning sector. Onset itself is, however, more localized spanning only about 1 hour local time. The average location of the peak intensity lbr 80 onsets was 65.9 + 3.5 CGMIat, 22.9 _+ 1.2 Mlt, whereas the average location of the AAF onsets was at 63.8 _+ 3.3 CGMIat, 22.9 _+ 1.1 Mlt. AAF onsets occur during time periods when the solar wind pressure is relatively high. These low-latitude wavelike onsets appear as precursors in the form of long-period magnetic pulsations (Pc 5 band) and frequently occur on the equatorward portion of the double oval distribution. AAFs brighten in conjunction with substom onset leading to the conclusion that they are a growth phase activity causally related to substorm onset. Precursor activity associated with these AAFs is also seen near geosynchronous orbit altitude and examples show the relationship between the various instrumental definitions of substorm onset. The implied mode number (30 to 135) derived from this work is inconsistent with cavity mode resonances but is consistent with a modified flute/ballooning instability which requires azimuthal pressure gradients. It is suggested that this instability exists in growth phase but that an additional factor exists in the premidnight sector which results in an explosive onset. The extended source region 'and the distance to the open-closed field line region constrain reconnection theory and local mechanisms for substom onset. It is demonstrated that multiple onset substorms can exist for which localized dipolarizations and the Pi 2 occur simultaneously with tail stretching existing elsewhere. Further, the tail can be less stretched at geosynchronous orbit during the optical auroral onset than during the precursor pseudobreakups. These pseudobreakups can be initiated by auroral streamers which originate at the most poleward set of arc systems and drift to the more equatorward main UV oval. Observations are presented of these AAFs in conjunction with low-and high-altitude particle and magnetic field data. These place the activations at the interface between dipolar and taillike field lines probably near the peak in the cross-tail current. These onsets are put in the context of a new scenario for substorm morphology which employs individual modules which operate independently or couple together. This allows particular substorm events to be more accurately described and investigated. • IZMIRAN, Moscow Region, Troitsk, Russia. 7937 7938 ELPHINSTONE ET AL.: OBSERVATIONS OF SUBSTORM ONSET existence of the growth phase was debated into the 1970s [McPherron, 1970; Starkov and Feldstein, 1971; Akasofu and Snyder, 1972; Feldstein, 1974]. It is now clear however that the growth phase does indeed occur and plays an important role in the substorm process [Pellinen and Heikkila, 1978; Kirkwood and Eliasson, 1990; Koskinen et al., 1992; Watanabe and lijima, 1993]. It also appears that dayside auroral activity can accompany the growth of electrojets prior to substorm onset [Elphinstone et al., 1991a]. Akasofu [1964] divided the expansive phase of a substorm into different parts with the first stage lasting about 5 min. This coincided with the time before the poleward motion began. If the poleward motion lasted only a few minutes this was termed a pseudo breakup. Later the substorm onset [Akasofu and Kan, 1982, p. 1315]
Substorm-induced energetic electron precipitation: Morphology and prediction
Journal of Geophysical Research: Space Physics, 2015
The injection, and subsequent precipitation, of 20 to 300 keV electrons during substorms is modeled using parameters of a typical substorm found in the literature. When combined with onset timing from, for example, the SuperMAG substorm database, or the Minimal Substorm Model, it may be used to calculate substorm contributions to energetic electron precipitation in atmospheric chemistry and climate models. Here the results are compared to ground-based data from the Imaging Riometer for Ionospheric Studies riometer in Kilpisjärvi, Finland, and the narrowband subionospheric VLF receiver at Sodankylä, Finland. Qualitatively, the model reproduces the observations well when only onset timing from the SuperMAG network of magnetometers is used as an input and is capable of reproducing all four categories of substorm associated riometer spike events. The results suggest that the different types of spike event are the same phenomena observed at different locations, with each type emerging from the model results at a different local time, relative to the center of the injection region. The model's ability to reproduce the morphology of spike events more accurately than previous models is attributed to the injection of energetic electrons being concentrated specifically in the regions undergoing dipolarization, instead of uniformly across a single-injection region.
Near‐Earth substorm onset: A coordinated study
1994
We present simultaneous satellite and ground-based measurements of a substorm. Throughout the initial substorm expansion, southward drifting arcs are observed poleward of the expanding substorm aurora, indicating two independent systems of particle precipitation. Freja passes the brightening onset arc in the topside ionosphere near the moment of the substorm onset, observing an Alfvdn wave, field aligned current and oxygen ion outflow. The substorm onset occurs at low magnetospheric L-shells, near the poleward edge of the region of trapped particles. The location and time for the substorm injection are confirmed by geostationary spacecraft together with magnetometers, allsky cameras and radar on the ground. We believe that the substorm onset may be triggered by modification of the oxygen content of the inner magnetosphere during the growth-phase caused by ionospheric ion outflow. Introduction The substorm growth phase is typically characterized by southward drifting auroral arcs and an enhancement of the electrojet currents [e.g., Pellinen and Heikkila, 1984]. At substorm onset the equatorwardmost discrete arc breaks up and starts a poleward auroral expansion [Akasofu, 1964]. A campaign of coordinated Freja/ground-based measurements was organized in March 1993. The ground instrumentation included all-sky cameras, magnetometers and a specially designed experiment for the EISCAT radar. We report observations during the late growth and early expansion phase of a substorm between 2040 and 2120 UT on March 2, 1993 and discuss their significance to the substorm onset mechanism. EISCAT and Freja are described by Baron,, [1984] and Andrg et al., [1993] respectively. Observations Auroral All-Sky Cameras: The four All-Sky Camera images (top and bottom) in Figure I are a representa
THEMIS observations of two substorms on February 26, 2008
Science China-Technological Sciences, 2010
Two substorms occurred at ~04:05 and ~04:55 UT on February 26, 2008 are studied with the in-situ observations of THEMIS satellites and ground-based aurora and magnetic field measurements. Angelopoulos et al. have made a comprehensive study of the 04:55 UT event. We showed detailed features of the two substorms with much attention to the first event and to the relationship between mid-tail magnetic reconnection (MR) and substorm activities. It was found that in the earlier stage of each substorm, a first auroral intensification occurred 2-3 min soon after the start of mid-tail MR, followed by a slow and very limited expansion. The auroral arcs were weak, short-lived, and localized, characterizing all features of a pseudobreakup. We regarded the first auroral brightening as the initial onset of the substorms. A few minutes later, a second stronger auroral intensification appeared, followed by quick and extensive expansions. It was interesting to note that the second brightening and related poleward expansion happened almost simultaneously (within a couple of minutes) with the onset of earthward flow and dipolarization in the near-Earth tail and other phenomenon of the substorm expansion phase. We thus regarded the second auroral brightening as the major onset of the substorms. Furthermore, it was seen that during the growth phase of the two substorms, the polar cap open flux Ψ kept increasing, while it quickly reduced during the substorm expansion and recovery phase. These variations of Ψ implied that the evolution of the two substorm expansion phases were closely related to MR of tail lobe open field lines. Analysis of substorm activities revealed that the two events studied were small substorms; while estimate of MR rate indicated that the MR processes in the two substorms were weak. The aforementioned observations suggested that mid-tail MR initiated the pseudobreakup first; the earthward flow generated by MR transported magnetic flux and energy to the near-Earth tail to cause the formation of SCW and CD, which induced near-Earth dipolarization and major auroral brightening, and eventually led to the onset of the substorm expansion phase. These results were clearly consistent with the picture of NENL and RCS models and supported the two step initiation scenario of substorms.
Global-scale electron precipitation features seen in UV and X rays during substorms
Journal of Geophysical Research, 1999
The Polar Ionospheric X-ray Imaging Experiment (PIXIE) and the ultraviolet imager (UVI) onboard the Polar satellite have provided the first simultaneous global-scale views of the patterns of electron precipitation through imaging of the atmospheric X-ray bremsstrahlung and the auroral ultraviolet (UV) emissions. While the UV images respond to the total electron energy flux, which is usually dominated by electron energies below 10 keV, the PIXIE, 9.9-19.7 keV X-ray images used in this study respond only to electrons of energy above 10 keV. Previous studies by ground-based, balloon, and space observations have indicated that the patterns of energetic electron precipitation differ significantly from those found in the visible and the UV auroral oval. Because of the lack of global imaging of the energetic electron precipitation, one has not been able to establish a complete picture. In this study the development of the electron precipitation during the different phases of magnetospheric substorms is examined. Comparisons are made between the precipitation patterns of the high-energy (PIXIE) and low-energy (UVI) electron populations, correlated with ground-based observations and geosynchronous satellite data. We focus on one specific common feature in the energetic precipitation seen in almost every isolated substorm observed by PIXIE during 1996 and which differs significantly from what is seen in the UV images. Delayed relative to substorm onsets, we observe a localized maximum of X-ray emission at 5-9 magnetic local time. By identifying the location of the injection region and determining the substorm onset time it is found that this maximum most probably is caused by electrons injected in the midnight sector drifting (i.e., gradient and curvature drift) into a region in the dawnside magnetosphere where some mechanism effectively scatters the electrons into the loss cone.
Ion composition of substorms during storm-time and non-storm-time periods
Journal of Atmospheric and Solar-Terrestrial Physics, 2002
The oxygen to hydrogen ratio (O + =H + ) is investigated for storm-time substorms (Dst = −195 nT) and for substorms in absence of storms (non-storm-time substorms) (Dst ¿ − 30 nT). For this study we use CRRES particle and electric ÿeld data as well as indicators for storm (Dst) and substorm (AE) activity and the IMF Bz-component from IMP-8. Injection of particles into the inner magnetosphere during storms and substorms is consistent with enhanced convection caused by large-scale storm-time or dipolarization dawn-dusk electric ÿelds in the near-Earth magnetotail. The strength of this convection is related to the magnitude and duration of the negative IMF Bz, which are both larger for storm-time substorms than for non-storm-time substorms. Non-storm-time substorms lead to only minor ring current enhancements at large L-values (L = 5:0-7.0), while storm-time substorms can contribute the main ring current. During storms the ring current maximum can move in as far as L = 3 from its quiet time position at L = 4:5. Storm-time substorms have O + =H + ratios of several 100%, whereas non-storm-time substorms show an O + =H + ratio between 15% and 65%. The obvious composition di erence between storm-time and non-storm-time substorms indicates that during storms one or a combination of the following factors play a role: (1) oxygen ions are preferentially accelerated=transported in the plasma sheet, (2) pre-storm conditions lead to a large reservoir of oxygen ions tailward of the oxygen trapping boundary, or (3) polar ionospheric out ow is enhanced leading to an increased convection source of oxygen ions.
Journal of Geophysical Research, 2008
1] Geosynchronous Los Alamos National Laboratory (LANL-97A) satellite particle data, riometer data, and radio wave data recorded at high geomagnetic latitudes in the region south of Australia and New Zealand are used to perform the first complete modeling study of the effect of substorm electron precipitation fluxes on low-frequency radio wave propagation conditions associated with dispersionless substorm injection events. We find that the precipitated electron energy spectrum is consistent with an e-folding energy of 50 keV for energies <400 keV but also contains higher fluxes of electrons from 400 to 2000 keV. To reproduce the peak subionospheric radio wave absorption signatures seen at Casey (Australian Antarctic Division), and the peak riometer absorption observed at Macquarie Island, requires the precipitation of 50-90% of the peak fluxes observed by LANL-97A. Additionally, there is a concurrent and previously unreported substorm signature at L < 2.8, observed as a substorm-associated phase advance on radio waves propagating between Australia and New Zealand. Two mechanisms are discussed to explain the phase advances. We find that the most likely mechanism is the triggering of wave-induced electron precipitation caused by waves enhanced in the plasmasphere during the substorm and that either plasmaspheric hiss waves or electromagnetic ion cyclotron waves are a potential source capable of precipitating the type of high-energy electron spectrum required. However, the presence of these waves at such low L shells has not been confirmed in this study. Citation: Clilverd, M. A., et al. (2008), Energetic electron precipitation during substorm injection events: High-latitude fluxes and an unexpected midlatitude signature,
Global Scale Electron Precipitation during Substorm Expansions
Astrophysics and Space Science Library, 1998
The Polar Ionospheric X-ray Imaging Experiment (PIXIE) and the Ultraviolet Imager (UVI) on the POLAR satellite have provided the first simultaneous global scale views of the patterns of electron precipitation through imaging of the atmospheric X-ray bremsstrahlung and the auroral UV emissions. While UVI responds to the total electron energy input, PIXIE responds only to the high energy (multi-keV) electron precipitation. During the substorm expansion phase, clear time delays occur between the electron injection at the nightside and the start of the precipitation on the morning/dayside. The observations are generally consistent with patterns previously deduced from ground-based and suborbital observations.
Magnetospheric substorms—definition and signatures
Journal of Geophysical Research, 1980
For many years, researchers have utilized definitions of the substorm phenomenon that are not consistent among one another, and this has created great difficulties in comparing the results reported in the literature by the various researchers. In August 1978, nine magnetospheric physicists active in the field of substorm research met in Victoria, British Columbia, Canada, to attempt to reach a consensus on an acceptable definition for a magnetospheric substorm. This paper reports the agreements reached at the Victoria workshop and presents an operational definition of the magnetospheric substorm and a critique of the various signatures by which researchers can identify the time sequence and spatial extent of the substorm. [ 1964]. Since that time a vast amount of research has been carried out that has greatly enhanced our understanding of the substorm phenomenon. However, it has become clear over this decade of research that the substorm phenomenon is more complex than it was originally envisaged. The scattered arrays of monitoring equipment have provided a complicated combination of spatial and temporal variations that are just now beginning to be appreciated. Furthermore, different groups have used different signatures to define the occurrence