M. Vellante

Solar activity dependence of geomagnetic field line resonance frequencies at low latitudes

[1] ULF field line resonance frequencies (f R ) of three different magnetic shells (L = 1.61, 1.71, and 1.83) have been monitored during a 4-year period (2001-2004) using a cross-phase analysis of magnetic measurements recorded at the South European Geomagnetic Array (SEGMA). We find that the variations of the daily averages of f R , which reflect changes in plasmaspheric mass density, follow the variations of the daily values of the 10.7-cm solar radio flux F 10.7 with an estimated time delay of 1-2 days. The analysis of selected events indicates that the sensitivity of f R to short-term (27-day) variations in the solar flux is the same as that for long-term (solar cycle related) variations. On the other hand, the results of the overall statistical analysis seem to indicate a lower sensitivity (by a factor of ∼2) of f R to short-term solar flux variations with respect to long-term variations. Geomagnetic activity effects and/or different solar variabilities of EUV and 10.7-cm flux for different timescales are suggested as a possible cause for such a difference. Experimental results are also compared with those provided by a physical-numerical model of the ionosphere-plasmasphere system. Last, we find some evidence for a slight annual variation in f R with an estimated summer/winter ratio of ∼ 1.1-1.2. The inferred corresponding annual variation in the equatorial mass density is in line with previous estimates for the European longitudinal sector as obtained from whistler measurements.

Recurrent flares in active region NOAA 11283

The authors wish to thank the referee for his/her very useful comments and suggestions, which led to a sounder version of the article. This research work has received funding from the European Commissions Seventh Framework Programme under the grant agreements No. 284461 (eHEROES project), No. 312495 (SOLARNET project), No. 606862 (F-Chroma project). This research work is partly supported by the Italian MIUR-PRIN grant 2012P2HRCR on The active Sun and its effects on Space and Earth climate and by Space Weather Italian COmmunity (SWICO) Research Program. The research by the KSU astronomy unit – A.E. and A.S.K. – was supported by King Saud University, Deanship of Scientific Research, College of Science Research Center.

Comparison of equatorial plasma mass densities deduced from field line resonances observed at ground for dipole and IGRF models

The technique to remotely sense the plasma mass density in magnetosphere using field line resonance frequencies detected by ground-based magnetometers has become more and more popular in the last few years. In this paper we examine the error that would be committed at low and middle latitudes (L < 4) in estimating the equatorial plasma mass density if dipole field lines are assumed instead of the more realistic representation given by International Geomagnetic Reference Field (IGRF) lines. It is found that the use of the centered dipole model may result in an error in the inferred density appreciably larger than what is usually assumed. In particular, it has a significant longitudinal dependence being, for example, greater than +30% in the Atlantic sector and about −30% at the opposite longitude sector for field lines extending to a geocentric distance of 2 Earth radii. This may result in an erroneous interpretation of the longitudinal variation in plasmaspheric density when comparing results from ground-based arrays located at different longitudes. We also propose simple modifications of the standard technique, such as the use of an effective dipole moment or the eccentric dipole model, which allow to keep using the dipole field geometry but with a significant error reduction.

Magnetospheric plasma density inferred from field line resonances: Effects of using different magnetic field models

The technique for remote sensing the plasma mass density in magnetosphere by geomagnetic field line resonances detected at ground-based stations is getting more and more popular after the establishment in the last few years of extended magnetometer arrays, such as the EMMA network recently formed in the framework of the EU FP-7 PLASMON project [1]. It is important therefore to quantify the level of accuracy associated to such technique. In this study we examine the effect of using different magnetic field models. First the equatorial plasma mass density estimates obtained using the dipole approximation are compared with those obtained using the IGRF model for low-mid latitudes. It is found that the use of the centered dipole model may result in an error in the inferred density appreciably larger than what is usually assumed. In particular it has a significant longitudinal dependence being, for example, greater than +30% in the atlantic sector and ~ -30% at the opposite longitude sector for field lines extending to a geocentric distance of 2 Earth radii. This may result in an erroneous interpretation of the longitudinal variation in plasmaspheric density when comparing results from ground-based arrays located at different longitudes. Simple modifications of the technique are proposed which allow to keep using the dipole approximation but with a significant error reduction. Then the results of using the T01 Tsyganenko model [2] are compared with those based on the IGRF model. With respect to previous studies we take into account the different equatorial crossing points of the IGRF and T01 field lines traced from a given ground position by considering reasonable radial gradients of the equatorial density. For average solar wind/magnetospheric conditions, mass densities computed using the IGRF model result to be moderately overestimated (less than 20%) for L values <; 4. The uncertainty obviously increases for higher L values and the bias may become negative for steep radial variations of the equatorial density. For storm-time conditions the error dramatically increases beyond L ~ 4, but may remain within ~ 20% for L <; 4 assuming radial variations of the equatorial density which are typical for such magnetospheric conditions. We also present an analysis of a real event using measurements provided by the European magnetometer network EMMA.

Some Aspects of the Low Latitude Geomagnetic Response under Different Solar Wind Conditions

We review some aspects of low latitudes (L≤2) geomagnetic field variations associated with magnetospheric pulsations as well as with continuous and impulsive variations of the solar wind (SW) pressure.

PLASMON: Data assimilation of the Earth's plasmasphere

The principal source and loss mechanisms in the Earths radiation belts are currently not completely understood. Loss rates are important since they determine the duration of exposure of satellites to enhanced radiation conditions during a geomagnetic storm. The dominant loss process is relativistic electron precipitation via resonant interactions with a variety of wave modes. These interactions are governed by the characteristics of the plasmasphere. Current models provide an inadequate representation of the spatial and temporal evolution of the plasmasphere. In situ measurements of the plasmasphere provide only local characteristics and are thus unable to yield a complete global picture. Ground based measurements, based on the analysis of Very Low Frequency (VLF) whistlers and Field Line Resonances (FLRs), are able to describe large sections of the plasmasphere, extending over significant radial distances and many hours of local time. These measurements provide electron number and plasma mass densities. PLASMON is a funded FP7 project between 11 international partners. PLASMON intends to assimilate near real time measurements of plasmaspheric densities into a dynamic plasmasphere model. The VLF whistler analyses will be conducted by automatic retrieval of equatorial electron densities using data from AWDAnet. Equatorial mass densities will be constructed from FLR measurements along meridional magnetometer chains. The resulting model will facilitate the prediction of precipitation rates. The predicted rates will be compared to observations from the AARDDVARK network.

Observation and Numerical Simulation of Cavity Mode Oscillations Excited by an Interplanetary Shock

Cavity mode oscillations (CMOs) are basic magnetohydrodynamic eigenmodes in the magnetosphere predicted by theory and are expected to occur following the arrival of an interplanetary shock. However, observational studies of shock-induced CMOs have been sparse. We present a case study of a dayside ultra-low-frequency (ULF) wave event that exhibited CMO properties. The event occurred immediately following the arrival of an interplanetary shock at 0829 UT on 15 August 2015. The shock was observed in the solar wind by the Time History of Events and Macroscale Interactions during Substorms-B and -C spacecraft, and magnetospheric ULF waves were observed by multiple spacecraft including the Van Allen Probes-A and -B spacecraft, which were located in the dayside plasmasphere at L∼ 1.4 and L∼ 2.4, respectively. Both Van Allen Probes spacecraft detected compressional poloidal mode oscillations at ∼13 mHz (fundamental) and ∼26 mHz (second harmonic). At both frequencies, the azimuthal component of the electric field (Eϕ) lagged behind the compressional component of the magnetic field (Bμ) by ∼90∘. The frequencies and the Eϕ-Bμ relative phase are in good agreement with the CMOs generated in a dipole magnetohydrodynamic simulation that incorporates a realistic plasma mass density distribution and ionospheric boundary condition. The oscillations were also detected on the ground by the European quasi-Meridional Magnetometer Array, which was located near the magnetic field footprints of the Van Allen Probes spacecraft.

Data assimilation of space-based and ground-based observations, and empirical models into a plasmasphere model

Summary form only given. The Earths plasmasphere is a region of dense plasma, originating in the ionosphere, extending nearly to geostationary orbit. The precise extent of the plasmasphere is dynamic, particularly during geomagnetic active conditions. Knowing the exact distribution of plasma in the plasmasphere is important as an input to coupled magnetospheric models. In particular, density gradients inside the plasmasphere and at the plasmapause, are important in controlling waves which are responsible for the growth and decay of the radiation belts. At the most basic level the plasmasphere can be described in terms of plasma exchange with the ionosphere and convection due to an imposed electric field. At that level plasmasphere modeling is relatively simple. However there is currently insufficient knowledge of the drivers, particularly the electric field, to model the plasmasphere boundaries at the most accurate level to provide sufficient quality inputs to wave and radiation belt models.

Correction to “Long‐period oscillations at discrete frequencies: A comparative analysis of ground, magnetospheric, and interplanetary observations”

[1] In the paper ‘‘Long-period oscillations at discrete frequencies: A comparative analysis of ground, magnetospheric, and interplanetary observations’’ by U. Villante et al. (Journal of Geophysical Research, 112, A04210, doi:10.1029/2006JA011896, 2007) the right panel in Figure 2c is wrong. Indeed, for the event A discussed in the paper, we reported the presence of simultaneous spectral enhancements at 1.3, 2.2, and 3.2 mHz in ground, magnetospheric, and solar wind observations (Figure 2b). In Figure 2c we correctly showed data filtered at f = 1.3 and 2.2 mHz in the left and middle panels but, by mistake, we included in the right panel, in correspondence to the 3.2 mHz fluctuations, a wrong plot with data filtered at f = 1 mHz instead of 3.2 mHz. We now provide Figure 2 with the correct plot. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, A08202, doi:10.1029/2007JA012552, 2007

ULF Wave Magnetic Measurements by CHAMP Satellite and SEGMA Ground Magnetometer Array: Case Study of July 6, 2002

We present the analysis of a Pc 3 geomagnetic pulsation event observed simultaneously by CHAMP and by the South European GeoMagnetic Array SEGMA (1.56 < L < 1.88) during the conjunction of July 6, 2002. Both compressional and transverse oscillations were identified in CHAMP magnetic measurements. A close correspondence between the compressional component and the ground signals is observed. At the same time the joint analysis of space and ground observations clearly indicates the occurrence of a field line resonance at L ✠ 1.6. A direct confirmation of the well known 90° rotation of the ULF wave polarization ellipse through the ionosphere is also provided.