Secondary Magnetic Reconnection at Earth’s Flank Magnetopause

We report local secondary magnetic reconnection at Earth’s flank magnetopause by using the Magnetospheric Multiscale observations. This reconnection is found at the magnetopause boundary with a large magnetic shear between closed magnetospheric field lines and the open field lines generated by the primary magnetopause reconnection at large scales. Evidence of this secondary reconnection are presented, which include a secondary ion jet and the encounter of the electron diffusion region. Thus the observed secondary reconnection indicates a cross-scale process from a global scale to an electron scale. As the aurora brightening is also observed at the morning ionosphere, the present secondary reconnection suggests a new pathway for the entry of the solar wind into geospace, providing an important modification to the classic Dungey cycle.

The Mie representation for Mercury’s magnetic field

The parameterization of the magnetospheric field contribution, generated by currents flowing in the magnetosphere is of major importance for the analysis of Mercury’s internal magnetic field. Using a combination of the Gauss and the Mie representation (toroidal–poloidal decomposition) for the parameterization of the magnetic field enables the analysis of magnetic field data measured in current carrying regions in the vicinity of Mercury. In view of the BepiColombo mission, the magnetic field resulting from the plasma interaction of Mercury with the solar wind is simulated with a hybrid simulation code and the internal Gauss coefficients for the dipole, quadrupole and octupole field are reconstructed from the data, evaluated along the prospective trajectories of the Mercury Planetary Orbiter (MPO) using Capon’s method. Especially, it turns out that a high-precision determination of Mercury’s octupole field is expectable from the future analysis of the magnetic field data measured by the magnetometer on board MPO. Furthermore, magnetic field data of the MESSENGER mission are analyzed and the reconstructed internal Gauss coefficients are in reasonable agreement with the results from more conventional methods such as the least-square fit.

Electric Mode Excitation in the Atmosphere by Magnetospheric Impulses and ULF Waves

Variations of vertical atmospheric electric field Ez have been attributed mainly to meteorological processes. On the other hand, the theory of electromagnetic waves in the atmosphere, between the bottom ionosphere and earth’s surface, predicts two modes, magnetic H (TE) and electric E (TH) modes, where the E-mode has a vertical electric field component, Ez. Past attempts to find signatures of ULF (periods from fractions to tens of minutes) disturbances in Ez gave contradictory results. Recently, study of ULF disturbances of atmospheric electric field became feasible thanks to project GLOCAEM, which united stations with 1 sec measurements of potential gradient. These data enable us to address the long-standing problem of the coupling between atmospheric electricity and space weather disturbances at ULF time scales. Also, we have reexamined results of earlier balloon-born electric field and ground magnetic field measurements in Antarctica. Transmission of storm sudden commencement (SSC) impulses to lower latitudes was often interpreted as excitation of the electric TH0 mode, instantly propagating along the ionosphere–ground waveguide. According to this theoretical estimate, even a weak magnetic signature of the E-mode ∼1 nT must be accompanied by a burst of Ez well exceeding the atmospheric potential gradient. We have examined simultaneous records of magnetometers and electric field-mills during >50 SSC events in 2007–2019 in search for signatures of E-mode. However, the observed Ez disturbance never exceeded background fluctuations ∼10 V/m, much less than expected for the TH0 mode. We constructed a model of the electromagnetic ULF response to an oscillating magnetospheric field-aligned current incident onto the realistic ionosphere and atmosphere. The model is based on numerical solution of the full-wave equations in the atmospheric-ionospheric collisional plasma, using parameters that were reconstructed using the IRI model. We have calculated the vertical and horizontal distributions of magnetic and electric fields of both H- and E-modes excited by magnetospheric field-aligned currents. The model predicts that the excitation rate of the E-mode by magnetospheric disturbances is low, so only a weak Ez response with a magnitude of ∼several V/m will be produced by ∼100 nT geomagnetic disturbance. However, at balloon heights (∼30 km), electric field of the E-mode becomes dominating. Predicted amplitudes of horizontal electric field in the atmosphere induced by Pc5 pulsations and travelling convection vortices, about tens of mV/m, are in good agreement with balloon electric field and ground magnetometer observations.

Characterization of the Pc5 frequency range power spectrum at low latitude in 22 years of geomagnetic field observations.

<p>An important aspect of the interaction between the solar wind (SW) and the Earth’s magnetosphere concerns the possible relationship between SW and magnetospheric fluctuations under different SW conditions. In recent investigations (Di Matteo and Villante, 2017,2018) we revealed the critical role of the analytical methods and the spectral analysis techniques in the identification of fluctuations between ≈1-5 mHz in the SW parameters as well as in the magnetospheric field measurements at the geostationary orbit and developed a new approach, based on the joint use of the Welch and the Multitaper methods, for a more robust identification of these oscillations in both regions. Here, we extend the analysis to ground measurements, analyzing 22 years of magnetic field measurements along the H and D components at low latitude (L’Aquila, Italy, λ≈36.3°, L≈1.6). We found that, in general, the much steeper spectrum of the geomagnetic fluctuations with respect to the ones estimated in the SW parameters and magnetospheric field, might deeply influence the identification of real events. We then examined, for the entire period, consecutive two hours intervals through the day during low geomagnetic activity conditions (Dst>-50), and, for each interval, we carefully evaluated the characteristics of the background spectrum. As a matter of fact, in the ≈1-5 mHz frequency range the spectral indices of both components typically range between -3.5 and -2 with a steeper spectrum in the night sector when the fluctuations power is lower. Simulations of red noise representations, with spectral indices similar to the observed ones, combined with the Sq variation show a systematic reduction of the rate of identification of real events up to ≈2 mHz.</p><p>Ref.</p><p>Di Matteo, S., and U. Villante, J. Geophys.Res. Space Physics, 122, 4905–4920, doi:10.1002/2017JA023936.</p><p>Di Matteo, S., and U. Villante, Journal of Geophysical Research: Space Physics, 123, doi.org/10.1002/2017JA024922.</p>

Correspondence between the ULF wave power spatial distribution and auroral oval boundaries

The world-wide spatial distribution of the wave power in the Pc5 band during magnetic storms has been compared with auroral oval boundaries. The poleward and equatorward auroral oval boundaries are estimated using either the British Antarctic Survey database containing IMAGE satellite UV observations of the aurora or the OVATION model based on the DMSP particle data. The “epicenter” of the spectral power of broadband Pc5 fluctuations during the storm growth phase is mapped inside the auroral oval. During the storm recovery phase, the spectral power of narrowband Pc5 waves, both in the dawn and dusk sectors, is mapped inside the auroral oval or around its equatorward boundary. This observational result confirms previously reported effects: the spatial/temporal variations of the Pc5 wave power in the morning/pre-noon sector are closely related to the dynamics of the auroral electrojet and magnetospheric field-aligned currents. At the same time, narrowband Pc5 waves demonstrate typical resonant features in the amplitude-phase latitudinal structure. Thus, the location of the auroral oval or its equatorward boundary is the preferred latitude for magnetospheric field-line Alfven resonator excitation. This effect is not taken into account by modern theories of ULF Pc5 waves, but it could be significant for the development of more adequate models.

Correspondence between the ULF wave power spatial distribution and auroral oval boundaries

The location of the auroral oval boundaries has been mapped onto the Pc5 wave power spatial distribution. The poleward and equatorward boundaries of auroral oval are estimated using either the BAS database based on UV observations of the aurora by the IMAGE satellite or the OVATION model based on the DMSP particle data. The epicenter of the spectral power of broadband fluctuations in the Pc5 band during storm growth phase is mapped inside the auroral oval. During the storm recovery phase, the spectral power of narrowband Pc5 waves, both in the morning and dusk sector, is mapped inside the auroral oval or around its equatorward boundary. This observational result confirms the effects earlier reported: the spatial/temporal variations of the Pc5 wave power in the morning/pre-noon sector are closely related to the location of the auroral electrojet and magnetospheric field-aligned currents. At the same time, narrowband Pc5 waves demonstrate typical resonant features in the amplitude-phase latitudinal structure. Thus, the location of the auroral oval (or its equatorward border) is the preferred latitude of magnetospheric field-line Alfven resonator excitation. This effect is not taken into account by modern theories of ULF Pc5 waves, but it could be significant for development of more adequate models.