Nov 1, 2000 - Proton precipitation related to Pcl pulsations. T. A. Yahnina, A. G. Yahnin. Polar Geophysical Institute, Apatity, Murmansk region, 184200, ...
GEOPHYSICAL RESEARCHLETTERS, VOL. 27, NO. 21, PAGES3575-3578,NOVEMBER 1, 2000
Proton precipitation related to Pcl pulsations T. A. Yahnina, A. G. Yahnin PolarGeophysical Institute,Apatity,Murmanskregion,184200,Russia
J. Kangas,J. Manninen Sodankyla Geophysical Observatory, Tahtelantie 112,FIN-99600Sodankyla, Finland
Abstract. By the analysis of one-year data from the lowaltitude NOAA satellite and on the basis of comparisonwith observationsof Pcl pulsations at Sodankyla Geophysical Observatory,Finland, we have for the first time found and describeda type of proton precipitationclosely related to Pcl. It is characterised by a localised(-•1ø of latitude)burst of both precipitating and locally trapped energetic(>30 keV) protons situated within the anisotropic precipitation zone. We found that intense Pcl on the ground can be observedat any distance(in MLT) from the footprintof satellite detecting the precipitation burst, but the probability of the Pcl observations strongly decreases with the distance. The frequency of the ground Pcl pulsations decreaseswith the increaseof the protonburst latitude. Thesefindings strongly confirm the idea that Pcl pulsations are the result of ioncyclotroninstability of energeticring currentprotons.
Introduction
Pcl pulsations observed on the ground and space are believed to be due to interactionbetweenenergeticions and cyclotron waves in the equatorial plane. One would expect that the pulsations should correlate with corresponding proton precipitation. But, to our knowledge,there is still no evidence of such a correlation. There are observations, which
show the correlation of Pcl with aurora.Mende et al. [1980] suggestedthat the aurora was due to proton precipitation. But, this suggestionhas been not proved yet. Some authors [e.g. Arnoldy et al., 1979; Pikkarainen et al., 1986] have observedriometer absorptionspikesproducedby the electron precipitation, which were correlatedwith ULF emissions in the Pcl range (IPDP). They, nevertheless,suggestedthat this precipitationwas stimulated by parasitic interaction between ion-cyclotronwaves and high energyelectrons. The aim of this paper is to present the ion precipitation patternwhich exhibits the signaturesof a close relation to the Pcl type pulsation. Data
For the present study the data from low-altitude NOAA satellites have been used. Among others, these satellites are equippedwith instrumentsfor measurements of particles with energy30 keV (MEPED) [Hill et al., 1985]. The energetic particles are measuredby two Copyright2000 by the AmericanGeophysical Union. Papernumber2000GL003763. 0094-8276/00/2000GL003763505.00
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detectors. At high latitudes (L>3) the orientation of the detectorsallows to observethe particles both within the loss cone (precipitating particles) and outside the loss cone (locally trapped particles). The data of geomagnetic pulsation observations at Sodankyla Geophysical Observatory (SGO, geographic coordinates67ø 22'N, 26ø 38' E; L=5.2) have been used as well. The pulsations are measured by search coil magnetometer and digitised data are routinely used for preparationof the daily spectrogramin the frequencyrange of 0-4 Hz.
Example of the event Below we describe a typical example of simultaneous particle and pulsationdata, on which our analysisis based. Plate 1 shows a daily spectrogram of electromagnetic emissionsin the frequencyrange of 0-4 Hz measuredat SGO on March 16, 1996. Several different types of geomagnetic pulsations are seen including Pcl, PiB, PiC, as well as SpectralResonanceStructurein the backgroundnoise (see, e.g. Kangaset al., 1998, Belyaevet al., 1999). Arrow marks an approximatetime of the NOAA-12 satellitepass(orbit 25028) over the northernpolar region. At this time intensetwo-band Pcl pulsations were registered at SGO. A closer look at
pulsationdata showedthat the signal was due to so-called structuredPcl or pearl pulsations. The particledatafor theNOAA-12 satellitearepresentedin Fig. 1. Typically the precipitationover polar region consists of two main zones: 1) isotropic precipitation zone, which is evidently due to the pitch-angle scatteringin the region of small magneticfield in the equatorial plane [e.g. Sergeevet al., 1983], and 2) anisotropiczone (equatorwardof isotropy boundary)wherethe trappedpopulation prevails.Sometimes, within the anisotropic zone a specific variation of proton fluxes is seen. They consist of short enhancementsof the trappedflux and, more seldom,of bursts of precipitating particles.In Fig. 1 suchspikesare seenapproximately at 1416 and 1432 UT, at 17.8 and 06.9 MLT, respectively. The precipitation is more often observed in the 30-80 keV channel,but occasionally they are also detected at energies more than 80 keV. Sometimes,the proton precipitation bursts associatewith energeticelectronprecipitation(as it seenafter 1432 UT). In some cases one can observe more than one protonflux enhancement. This is also shownin Fig. 1, where there are two closely located increasesof the flux, one of which (in both MLT sectors)is almost isotropic. There are no enhancementsin the low-energyprecipitation (data obtained from TED are shown in two panels in the bottom) correlated with the varyingenergeticparticlefluxesdescribedabove.
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YAHNINA ET AL.: PROTONPRECIPITATIONRELATEDTO Pcl PULSATIONS
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Plate1. Exampleof daily spectrogram plot (March 16, 1996) showingdifferenttypesof the ULF electromagnetic emissionsin the rangeup to 4 Hz. Pcl pulsationsare seenat 12-17 UT. Arrowmarksan approximatetime of NOAA-12 satellitepolar cap crossing,data from which are shownin Fig. 1. Similar particle features have been also observed on several other NOAA-12 passes on March 16, 1996 (approximatelyat 1340 UT, 1520 UT, and 1557 UT). All of
burstswere detectedonly in 7% of the cases.Variations of the trapped flux are not temporal; they can be observed in the sameMLT sectoras long as severalhours,and simultaneously themoccurred duringthe Pcl interval.Therewerealsopasses at differentMLTs. But precipitatingprotonsare rarely seenon when neither Pcl pulsations nor proton events were subsequentsatellite passes. registered.This gave us an idea that considered localised In this study,to increasethe statisticsof the precipitation proton enhancements(LPE)might closely relate to the Pcl bursts, we analyse observations obtained during the whole waves.Below we will confirmthe relationship statistically. year 1996. Altogether 213 LPEs were registered in both hemispheres.Fig. 2 (upper panel) presentsthe number of the LPEs during every month of the year. At the middle of the Proton enhancement characteristics and Figure the total time of Pcl observations(in hoursper month)
their comparison with ground Pcl
is shown. Both LPE and Pc l exhibit
similar
annual variation
Somecharacteristics of the proton enhancementsdescribed that in turn correspond to the variation of geomagnetic above have been studied by Yahnina et al. [1998]. These activity as characterisedby Dst-index shown in the bottom. consideration showed that in fact the authorsanalysedthe NOAA-6 and TIROS satellite data for 2 0 More detailed daysin August 1979. They foundsuchphenomenaduring the phenomena under study appeared during the Dst-index recovery phases of geomagnetic storms. Typically only recovery. We checked if Pcls were observed during every trapped flux variations were observed; the precipitation LPEs. The resultis that 91 percentsof the proton bursts were associatedwith Pcl pulsations registered at SGO. The Pcls observedin the Northern Hemispherehave been divided into 16March 1996 NOAA-12 Orbit25028 three groupsaccordingly to their intensity. (The intensity in 1.0E+6 1.0E+5 1.0E+4 1.0E+3 1.0E+2
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spectrograms).The group of very weak intensity consists of 39 events, second group (medium intensity) contains 22 events, and third group -55 intense Pcl events. The cases when Pcl was not observed during the LPE (11 events) were addedto the group of very weak pulsations.Fig. 3 shows how the occurrencerate of Pcl depends on the distance between observingstation (SGO) and satellite footprint. As a measure
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