EMIC Waves and Geomagnetic Storms

R. E. Erlandson1 and A.J. Ukhorsky 1,2

1 The Johns Hopkins University Applied Physics Laboratory

erlandson@jhuapl.edu

1,2 Moscow Institute of Physics and Technology

 

GEM '99 Snowmass Summer Workshop

June 21 - 25, 1999

 

Abstract

In this presentation we present results on a study that investigates the occurrence of EMIC waves during geomagnetic storms. We use two different methodologies in this study.

 
  1. Use a statistical study to address the occurrence of EMIC waves. Focus on magnetic latitude dependence of EMIC waves and occurrence during large magnetic storms.
  2. We conduct case studies to study waves occurrence and properties during large magnetic storms (Dst < -100 nT).

 

Motivation
  1. Are ground and off-equatorial (MLAT>30º) satellite based EMIC Wave (Pc 1) observations a reliable indicator of EMIC wave occurrence during geomagnetic storms?
  2. Thorne and Horne [JGR, 102,14155, 1997] present a scenario where initially during a storm the energy density of H+ is enhanced and generates EMIC waves. As O+ concentration increases during a storm (upflowing ion source) the EMIC waves (f>fO+) are absorbed by O+. As O+ anisotropy increases wave growth is possible at f<fO+. Waves with f<fO+ may propagate to the ground.

 

Magnetic Latitude Dependence of EMIC Wave Occurrence

  EMIC Wave Normalized Frequency (-10<MLAT<10)

  EMIC Wave Normalized Occurrence vs L

  EMIC Wave Normalized Occurrence vs MLAT

 

Magnetic Latitude Dependence Summary (Part I)
  1. The occurrence of EMIC waves along a given L-shell is strongly dependent on magnetic latitude (MLAT). The peak rates are found at the equator and fall off sharply at ± 16º MLAT. It is found that 75% of equatorially generated EMIC waves near L=3-5 do not reach magnetic latitudes > 16.
  2. The drop in occurrence off the equator can be caused by EMIC waves generated in the unguided mode or absorption of the waves by heavy ions. The later process occurs for waves with frequencies above the heavy ion gyrofrequency that propagate away from the equator. These waves encounter the bi-ion hybrid frequency (occurs just above heavy ion gyrofrequency) off the equator. When the wave frequency matches the heavy ion gyrofrequency, the waves are either reflected back towards the equator, tunnel through the stop band, or become absorbed by heavy ions in the plasma.
  3. The most likely process involves EMIC wave loss via resonant interactions with oxygen ions off the equator. The reason is that we observe the occurrence rate drop for waves both above and below the He+ gyrofrequency (unguided mode waves occur above the He+ gyrofrequency). This point, however, is still under investigation.
  4. IMPLICATION: Not all equatorially generated EMIC waves reach the ground. This makes it unreliable to use ground based observations to determine the occurrence of these waves during geomagnetic storms.
  5. QUESTION: Are the DE-1 and CCE statistical study results consistent? The occurrence rates at low-L observed in the DE-1 and CCE studies are consistent (.1-1% occurrence at 3<L<5). The occurrence rates at high-L (L>7) are not. DE-1 observed low occurrence rates (1%) and CCE observed high rates (10-20%). The main difference is that DE-1 sampled high-L regions at high magnetic latitudes (MLAT > 30)

 

 

II. EMIC Wave - Storm Case Studies:

 

Motivation Part II
  1. Rate of ring current recovery during early recovery phase occurs faster than expected when considering charge-exchange loss mechanisms. The question is, do EMIC contribute to ring current loss processes during the early recovery phase.
  2. The question is, do observations support the occurrence of these waves during the early and later recovery phase of geomagnetic storms.

 

Storm Case Studies

These plots contain Dst (top panel), L-value of CCE (dashed line) and DE-1 (solid line) in middle panel, and MLT of CCE and DE-1 in bottom panel. The green symbols represent EMIC wave events (bottom 2 panels). The green symbols in top panels represent the wave frequency and the blow symbols represent the wave power. The diamond represents CCE and the * represents DE-1.

  DST - EMIC Wave Occurrence Plot (Storm 85028)

  DST - EMIC Wave Occurrence Plot (Storm 86286)

  DST - EMIC Wave Occurrence Plot (Storm 86330)

 

DE-1/CCE Storm Study Summary:
  1. We used DE-1 and AMPTE/CCE data within 15 degrees of the magnetic equator to investigate the occurrence of EMIC waves during large geomagnetic storms (DST < -100 nT).
  2. A total of 4 out of 45 storms that occurred from 81-89 have been studied. The results are promising but inconclusive
  3. Statistical Study Results Indicate that the occurrence of EMIC waves during storm intervals (DST<-100) is 4 times larger than during quiet times.

 

 

GEM 99 Storm Intervals (Sodankayla Pc 1 Data)*

May 1997 Storm

March 1998 Storm

June 1998 Storm

* Courtesy of SOD Observatory; K. Mursula and R. Rasinkangas (Univ. of Oulu)

 

Contact Information: erlandson@jhuapl.edu

 

Links: Return to GEM99 Poster Page

Last Revised: June 18, 1999

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