Observing ion cyclotron waves

M. R. Lessard, M. Widholm,P. Riley, H. Kim

M. J. Engebretson

University of New Hampshire

Augsburg College

NSF Workshop on Small Satellite Missions for Space Weather and Atmospheric Research

George Mason University, Arlington Campus, VA

Pressure pulses, EMIC waves and the ring current

1. Electromagnetic Ion Cyclotron (EMIC) waves first observed in the vicinity of the plasmapause, associated with anisotropic proton distributions (10’s of keV) and increased levels of cold plasma.

2. Likely plays a role in thermalizing ring current protons immediately following magnetic storms [Cornwall et al., 1970; Lyons and Thorne, 1972; Cornwall et al., 1970; Lyons and Thorne, 1972].

3? Satellite studies confirm an increase occurrence of EMIC waves during storms [Bräysy et al.,1998 and Erlandson and Ukhorsky, 2001]. Can contribute significantly to ring current losses during recovery phase.

4? Several studies show ground observations of EMIC waves associated with pressure pulses [Olson and Lee, 1983; Kangas et al., 1986 Arnoldy et al., 1988; Arnoldy et al., 1996]. Recent work shows ground/satellite observations of these waves with precipitation of protons with several keV energy [Arnoldy et al., 2005].

Pressure pulses, EMIC waves and the ring current

EMIC wavegeneration region

Energetic particlePrecipitationregion <-propagation

EMIC wave propagation and radiation belt electrons

Ground observations (Engebretson et al, manuscript under review) show:

1. Narrowband Pc 1-2 waves are rarely if ever observed on the ground during the main and early recovery phases of magnetic storms.

2. As storm recovery progresses, occurrence of Pc 1-2 waves increases, at first during the daytime and afternoon sectors, but at all local times typically by days 3 or 4.

Why waves are seen in space during onset and main phase and consistently not seen on the ground is not understood - likely implication is that some reflection and/or absorption of these waves takes place away from the equator. Or, perhaps the satellite-based observations are, in fact, atypical.

EMIC wave propagation and radiation belt electrons

May cause precipitation of 10-50 MeV electrons. Have also been identified as major contributors to “parasitic” electron loss into the atmosphere at MeV energies [e.g., Lyons and Thorne, 1972; Summers and Thorne, 2003, Summers et al., 2004].

SAMPEX measures energetic particles at 520 to 670 km, but has no information regarding waves and, especially, has no information about waves near the equator!!

In order to understand the relationship between storms, EMIC waves and the radiation belts, observations of precipitating particles need to made in conjunction with wave activity near the equator.

Example: EMIC waves recorded at Halley Station in Antarctica

L=4.2, MLT = UT - 3h http://dabs.nerc-bas.ac.uk/qlook/Qlook_Catalog.html

Fundamental space weather objectives are contained in the relationship between:

Ring current particles….. EMIC wave generation/propagation….. Radiation belt electrons

Needed measurement:

1. Explore the region near and away from the equator to observe the generation and propagation of EMIC waves.

2. Simultaneously, observe regions lower in altitude in order to measure precipitating energetic electrons.

3. Possible satellite configurations include “string of pearls”, identical orbits at different MLT, or a combination of both. Others???

Measurement objectives

Measurement techniques 1

Fluxgate magnetometer

Issues:

1. Dynamic range versus resolution are usually traded off such that resolution suffers. Eg., Cluster magnetometer has a noise floor of ~5 pT √Hz, but resolves only 125 pT in near-Earth regions (to measure +/- 1024 nT).

This resolution is typical of existing and previous satellite missions (including ST-5), but falls far short of the ~pT resolution in ground-based instruments.

2. Ultimate noise floor (5 pT √Hz for Cluster, 8 pT √Hz for MMS) may not be optimal for EMIC waves.

Measurement techniques 1

Fluxgate magnetometer - possible solution

Subtract a baseline from the raw signal. Note that the subtraction must be accurate to within ~1 part in 106.

Digitize the residual signal separately from the DC signal.

Signals the order of a >5 pT √Hz may be detectable.

Measurement techniques 2

Induction coil magnetometer for ground-based applications. Uses 16 spools, with 10,000 turns on each spool to achieve ~1 pT minimum discernable signal (or better). Core is high-permeability.

Measurement techniques 2

Improving perfomance - how to shrink an existing design (objective is to detect ~1 pT or less):

1. Update preamp, reduce noise x4. This would reduce the number of turns required by x4. If we are willing to accept a resolution that is ‘worse’ by x2, can use 20,000 turns in existing design. Total mass of this sensor would be ~0.8 kg.

2. Use higher performance mumetal, reduce core mass by ~20. The mumetal in existing design was not intended for space flight and better materials exist. Some concern about saturation (solution is to apply a DC bias to the core). Expected mass would be ~.4 kg.