High Latitude Outer Radiation Belt Boundary Dynamics in Comparison with NOAA POES Polar Ovals V

WDS'14 Proceedings of Contributed Papers — Physics, 331–336, 2014. ISBN 978-80-7378-276-4 © MATFYZPRESS

High Latitude Outer Radiation Belt Boundary Dynamics in Comparison with NOAA POES Polar Ovals V. O. Barinova, V. V. Kalegaev, I. N. Myagkova, M. O. Riazantseva D.V. Skobeltsyn Institute of Nuclear Physics (SINP) Of Moscow State University (MSU), Moscow, Russia.

Abstract. This paper describes the geometry and the behaviour of the Earth’s outer radiation belt polar boundary at the altitudes between 500 and 1000 km from the Earth surface in dependence of universal time and geomagnetic activity expressed by the Dst-index. The quantitative model which was built earlier for the Northern hemisphere in quiet conditions using the Coronas-Photon data measured in 2009 is upgraded in this work using Meteor-M No.1 data for being usable during periods of disturbed magnetosphere. Both hemispheres are studied now. Observations are taking place at diﬀerent altitudes. The outer radiation belt boundary is compared with the Polar Oval using data of NOAA POES for the period of time from September 2011 till July 2012.

Introduction A radiation belt is a structure formed by the trapped radiation. The Earth’s outer radiation belt mostly contains electrons. For the ideal dipole magnetic field it can be easily described by two surface equations [van Allen, 1962]. However, the observed boundary differs from the calculated one. The outer radiation belt high-latitude boundary can be observed onboard polar low-altitude orbit satellite using electron fluxes measurements. There are two points at the time profile of the electron fluxes near the timestamp when the satellites latitude achieves its maximum or minimum at every orbit where the abrupt decrease or increase of the flux changes to approximate constant value. These two points are at the time when the satellite orbit crosses the boundary of the outer radiation belt. If the Earth field was the ideal dipole field, the boundary at every altitude would be the circle and could be calculated using the known formula [van Allen, 1962]. The goal of this work was to describe the observed shape of the boundary and variations of its position. It is well known that the outer radiation belt particle fluxes abruptly change during geomagnetic storms [Kuznetsov et al., 2007] which are induced either by arrival of coronal mass ejections [Panasyuk, 2004; Gopalswamy, 2006] or high-speed fluxes of the solar wind [Li, 2005]. The size of the trapped radiation region is also very sensitive to the solar wind speed and geomagnetic activity [Kuznetsov et al., 2007]. During the magnetic storm main phase of Coronal Mass Ejections(CME) and (Co-rotating Interaction Region) (CIR)-driven storms when the Dst- index reaches the minimum, the locations of the outer boundary move to L = 4 and L = 5.5, respectively [Yuan, 2011]. Some peculiarities of outer radiation belt dynamics (position of maximum fluxes of relativistic electrons, location of rapid enhancements of the electron fluxes) were studied by Myagkova [2010]; Tverskaya et al. [2007, 2008]. Building of the Earth’s outer radiation belt boundary model can be divided into several stages. The disturbed periods were excluded first and it made it possible to build the statistical model of the Earth’s outer radiation belt. It based on the observations only and the best picture was seen when built in Geodetic coordinates with the fixed earth position. The other coordinate systems we tried, especially geo and other magnetic coordinates blurred the boundary, made it became wider and its shape became strictly round due to the Earth rotation and the rotation of the magnetic poles with it. So we decided to build the model in dependence on the universal time only using Coronas-Photon satellite data [Barinova et al., 2011]. Coronas-Photon satellite operated from March till November in 2009. The channel of the electrons > 0.2–1 MeV was selected as the channel with the lowest energy of the electrons, the

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Figure 1. Northern Hemisphere. Projected crossings of the outer radiation belt boundary by the Coronas-Photon (black) and Meteor-M No.1 (white) orbits. Electrons 0.2–1 MeV onboard Coronas-Photon and Electrons > 0.1 MeV onboard Meteor-M No.1. highest electron fluxes values in the radiation belt and as the result — the most distinguishable boundary of the radiation belt at the time profile graph. An algorithm of automatic boundary detection using particle fluxes time profile was developed in the previous paper by Barinova et al. [2011], a database of boundary crossings by the satellite orbit was created and the model of the outer radiation belt boundary dynamics for the quiet period, when it depends only on the universal time, was constructed. This boundary cannot be described as an absolytely thin line with no width, a mathematical abstraction which separates the radiation belt and polar cap regions but it was decided to approximate it with an absolutely thin average curved line at the satellite altitude using the elliptic formula as a base. The new goal was to explore the dynamics of the outer radiation belt high-latitude boundary using the magnetic storm database to upgrade the model for calculating the position of the high latitude boundary for the periods of disturbed magnetosphere.

Instruments comparison It was impossible to use data of the Coronas-Photon satellite because it stopped operating in 2009, the 30th of November. But since both the Coronas-Photon solar observatory and the Meteor-M No.1 satellites had circular polar orbits (Coronas-Photon: 550 km altitude, 82.5◦ inclination; Meteor-M No.1: 832 km altitude, 101.3◦ inclination), it was possible to use Meteor- M onboard instruments to monitor the Earth’s outer radiation belt at low altitudes. The MSGI instrument installed onboard the Meteor-M No.1 spacecraft consists of a number of semiconductor and scintillation detectors and it is used to measure energetic particle fluxes (0.1–13 MeV electrons and 1–260 MeV protons) [Meteor-M No.1 web-page, 2012]. The most similar to the previous channel is electrons > 0.1 MeV measured by the MSGI instrument. Fortunately there was a month when Coronas-Photon and Meteor-M No.1 were in operating together [Barinova et al., 2011]. But since the altitude of the orbit of Meteor-M No.1 differs from the Coronas-Photon one it was necessary to make a special projection of the Coronas-Photon points to the altitude of Meteor-M No.1 orbit using the equation of the dipole field line: (We can use the dipole cause the accuracy of the model cannot be more than the accuracy of the measurements)

r = L cos2(φ). (1)

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Figure 2. Time proﬁle of the particle ﬂuxes. Data was averaged to 30 seconds to better view and processing.

Where φ is the geomagnetic latitude, r is the distance to the Earth center measured in the Earth radii and L is an L-shell (Mcilwain coordinate). And the latitude of the searched point of crossing of the outer radiation belt boundary by the Coronas-Photon for the Meteor-M altitude can be found (in degrees):

180 rrmet φmet = ± arccos(cos(φcor) ). (2) π rcor Longitude stays the same. Figure 1 shows the visual result of the data comparison.

Disturbed periods Magnetic storm is often caused by CME connected with solar flare and is accompanied by the solar cosmic rays event. In this case both polar caps are filled with the solar particles and the detection of the boundary becomes impossible. But even when it’s possible sometimes it’s easier to process some periods manually then an algorithm. Figure 2 shows a stormy period but caps have just started to be fulfilled so boundaries can be detected. Over 40 storms, 8 of which were strong (Dst < −100 nT), were observed and studied since November 2009 till June 2012. The result was added to the database of the outer radiation belt boundary crossings by the satellites orbits. Since Meteor-M No.1 has Solar synchronized orbit, this database cannot be effectively split hourly to determine the UT-effect, but it had already been included into the model in the previous work [Barinova et al., 2012]. Now it became possible to split this database by Dst index because it varies from −137 to +80 nT. (In 2009 −40 < Dst < 40 nT except a few times). Figure 3 shows the plot of all the data in the polar geographical coordinate system. Each point is a place where the satellite crosses the outer radiation belt high-latitude boundary in various moments of time. The color of the dot is selected using the Dst scale: the less Dst, the darker color is at the point.

The model upgrade By minimizing the RMS deviation, the equation of the ellipse describing the average outer radiation belt boundary was fitted for different intervals of Dst values. The linear dependency of the length of the major axis of the ellipse on DST was found. Non-linear terms are out of the measurement and data processing accuracy. The area filled with points is determined by the structure of geomagnetic field. It is prolate and shifted as expected. The width of this statistical boundary is about 5◦ which shows a good quality of boundary detection algorithm. The width is seen at the picture and also measured by taking average values at the various longitudes. According to the Earth’s rotation each meridian

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Figure 3. Dst dependency: every dot painted in a color in Dst scale.

Figure 4. Ellipses are built in accordance with the data processed. became subsolar during one day and the tail current pulled the boundary to the opposite side, so the shift should be observed at the different longitudes. It is assumed in the model that the orientation and eccentricity of the ellipse does not change, and the center moves by the small ellipsoidal trajectory symmetrical around the magnetic pole. It is expected that the night shift should be larger in disturbed than in quiet period, but further investigations are needed. Using the main equation from Barinova et al. [2012] we now can find the first terms, assuming the linear dependencies. Non-linear terms are not necessary since the accuracy of the model cannot be more than the accuracy of the measurements.

Dst 2π 2 2 X − sin UT + Y − Dst p(1 − ecc2) cos UT 2π · 1 = CDst 24 CDst 24 (1−ecc2) 2 Dst = R − CDst , (3) where R is the radius (semiaxis) along the X axis and its average value is about 22◦, while Dst/CDst = r average value is about 1.5 degrees in quiet periods and achieves 6–7◦. CDst = 18.0. Northern ecc = 0.6. Southern ecc = 0.1.

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Figure 5. Position of the auroral oval extrapolated from NOAA measurements and boundary model (black ellipse). Quiet period.

Figure 6. Position of the auroral oval extrapolated from NOAA measurements and boundary model (black ellipse). Magnetic storm.

Comparison with NOAA POES polar oval position NOAA POES [NOAA POES web-apge, 2014] satellites are operating at approximately the same altitude as Meteor is (The initial orbital parameters of NOAA 18 were period 102 min, apogee 866 km, perigee 847 km, and inclination 98.74). They collect the data of the power ﬂuxes and create statistical patterns which are color coded on a scale from 0 to 10 ergs·cm−2·sec−1. NOAA is an American (NOAA) weather satellite series that carry several weather-related instruments and Space Environment monitors to observe the Earth in visible, infrared and ultraviolet light and to monitor the energetic electrons and protons in the radiation belt. The plots in Figures 5 (2011-Oct-27 10:23) and 6 (2011-Oct-25 02:00) show the extent and position of the auroral oval at each pole, extrapolated from measurements taken during the most recent polar pass of a NOAA POES satellite in quiet period and in the storm. One can see that High Latitude Boundary of the Outer Radiation Belt corresponds well to the equatorward boundary of the auroral oval but is located at lower latitudes. The auroral light is produced by precipitating particles of energies smaller than in the radiation belt. The outer radiation belt boundary was expected to be at lower latitudes than this oval, which was successfully conﬁrmed by the investigation during quiet and non quiet conditions.

Summary Dynamics of High Latitude Boundary of the Outer Radiation Belt in Disturbed periods was investigated using the data of the Meteor-M No.1 satellite. The model of its form and location has been constructed in dependence of the UT and geomagnetic conditions. For both hemispheres the dependencies on UT and Dst are obtained to be the same as expected but the

335 BARINOVA ET AL.: HIGH LATITUDE OUTER RADIATION BELT BOUNDARY DYNAMICS shapes of the boundaries are diﬀerent due to the Earth’s internal magnetic structure. Dynamics of the boundary can be represented as an ellipse which is moving due to Earths rotation (Ut-eﬀect) shift to the night side due to the magnetospheric noon-midnight asymmetry and equatorward expansion due to geomagnetic activity. Equatorward expansion of the HLB ORB for the moderate magnetic storms (Dst ≈ −100 nT) is about 10◦.

Acknowledgments. The present work was supported by the RFFI grant No. 12-05-00984- All data are collected on site of the Space Monitoring Data Center of SINP MSU smdc.sinp.msu.ru and updated daily.

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