Joachim Vogt

Modeling impacts of geomagnetic field variations on middle atmospheric ozone responses to solar proton events on long timescales

[1] Strength and structure of the Earth’s magnetic field control the deflection of energetic charged particles of solar and cosmic origin. Therefore variations of the geomagnetic field occurring on geological timescales affect the penetration of charged particles into the atmosphere. During solar proton events (SPEs) the flux of high-energy protons from the Sun is markedly increased. In order to investigate the impact of SPEs on the middle atmospheric ozone on longer timescales, two-dimensional atmospheric chemistry and transport simulations have been performed using simulated time series of SPEs covering 200 years. Monte Carlo calculations were used to obtain ionization rates, which were then applied to the atmosphere under the consideration of different shielding properties of the geomagnetic field. The present-day magnetic field configuration and four other scenarios were analyzed. For the first time, field configurations representing possible realistic situations during reversals have been investigated with respect to SPE-caused ozone losses. With decreasing magnetic field strength the impacts on the ozone are found to significantly increase especially in the Southern Hemisphere, and subsequently, the flux of harmful ultraviolet radiation increases at the Earth’s surface. The ozone destructions are most pronounced in the polar regions, and for some field configurations they exceed the values of ozone hole situations after large SPEs. In contrast to ozone holes the depletions due to SPEs are not restricted to winter and spring times but persist into polar summer.

Shear velocity profiles associated with auroral curls

Optical observations using high-resolution television cameras frequently show that auroral curls are associated with shear velocities in the apparent optical flow. The present study examines in detail one particular curl system event which happened to yield sufficient resolution to determine the fine structure of velocity and vorticity profiles by means of a new analysis technique. Those observations of curl system evolution are contrasted with large velocity shear events where small-scale quasiperiodic distortions were subject to sudden decay rather than development into vortices. The results are discussed in light of an electrostatic picture of auroral acceleration and the Kelvin-Helmholtz instability model. We suggest that the latter cannot fully explain the nonlinear phase of the observed curl system event and that curl models should take auroral acceleration processes into account.

A model for the reflection of Alfvén waves at the source region of the Birkeland current system: The tau generator

The source region of the Birkeland current system, also called the generator of the auroral current circuit, is probably located in the outer equatorial magnetosphere on closed magnetic field lines. Alfven waves have often been suggested as carriers for field-aligned currents, momentum, and electromagnetic energy which is converted to particle kinetic energy and ohmic heat at the polar end of the auroral flux tubes. In this report we present a model for the interaction of Alfven waves with the plasma in the generator region: the tau generator. We show that existing concepts like current and voltage generators correspond to extreme values of the intrinsic parameter τ. A frequency dependent reflection coefficient can be found which has considerable effect on the evolution of magnetospheric oscillations. Furthermore, we discuss the physical significance of the parameter τ and relate it to some measure for the effective height of the generator region.

Persistent quasiperiodic precipitation of suprathermal ambient electrons in decaying auroral arcs

An analysis of ground-based images and rocket-borne electron data from the Physics of Auroral Zone Electrons II rocket experiment suggests that suprathermal ambient electrons, precipitating in field-aligned bursts, may play an active role in regulating the decay of auroral potential structures. A set of discrete arcs were observed to form in quasiperiodic succession behind a passing westward traveling surge. The nascent arcs faded rapidly, losing 90% of their luminosity within 15 s. The fading then abruptly stopped at a brightness consistent with a peak electron energy of ∼1.5 keV. Rocket-borne measurements of energetic electrons over the residual arcs 2 min later revealed a superposition of hot isotropic and cold ionospheric electrons, the latter precipitated in dispersive periodic (3-5 Hz) bursts from a source altitude of 3000-6000 km. A combined analysis of the optical and particle data showed that the FAB component (1) was present for at least 1 min, (2) precipitated in spatial scales of ∼10 km and (3) carried a number flux comparable to that of the overlying isotropic population. Bulk plasma parameters calculated in the vicinity of the fading arcs revealed an inverse relationship between parallel potential drop and number flux carried by the field-aligned population.

Magnetosphere‐ionosphere coupling of global Pi2 pulsations

Global Pi2 pulsations have mainly been associated with either low/middle latitudes or middle/high latitudes and, as a result, have been treated as two different types of Pi2 pulsations, either the ...

The Sun, geomagnetic polarity transitions, and possible biospheric effects: review and illustrating model

The Earth is embedded in the solar wind, this ever-streaming extremely tenuous ionized gas emanating from the Sun. It is the geomagnetic field which inhibits the solar wind plasma to directly impinge upon the terrestrial atmosphere. It is also the geomagnetic field which moderates and controls the entry of energetic particles of cosmic and solar origin into the atmosphere. During geomagnetic polarity transitions the terrestrial magnetic field decays down to about 10% of its current value. Also, the magnetic field topology changes from a dipole dominated structure to a multipole dominated topology. What happens to the Earth system during such a polarity transition, that is, during episodes of a weak transition field? Which modifications of the configuration of the terrestrial magnetosphere can be expected? Is there any influence on the atmosphere from the intensified particle bombardment? What are the possible effects on the biosphere? Is a polarity transition another example of a cosmic cataclysm? A review is provided on the current understanding of the problem. A first, illustrating model is also discussed to outline the complexity of any biospheric reaction on polarity transitions.

Local time resolved dynamics of field-aligned currents and their response to solar wind variability

Using 10 years of CHAMP measurements condensed into the empirical model of field-aligned currents through empirical orthogonal function analysis, the dynamics of field-aligned currents (FACs) is modeled and studied in separate magnetic local time (MLT) sectors. We investigate the distributions of FAC intensity and latitude and evaluate their predictability in terms of geospace parameters which are ranked according to their relative importance measured by a multivariate regression procedure. The response time to changes in solar wind variables is studied in detail and found to be much shorter for dayside FACs than on the nightside (15–25 min versus 35–95 min). Furthermore, dayside FACs can be parameterized more accurately: R2 values maximize greater than 0.7 for FAC latitude and greater than 0.3 for FAC intensity, whereas the corresponding values on the nightside are smaller than 0.3 and 0.15, respectively. The results support the separation between directly driven coupling processes acting on the dayside and unloading processes controlling the nightside. In addition, the MLT-resolved standardized regression coefficients suggest that (1) FAC latitude is affected most significantly by the transpolar potential, substorm evolution, solar activity as represented by the F10.7 index and its square, and the dipole tilt; (2) Region-1/2 current intensity is controlled most efficiently by substorm evolution, IMF Bz and IMF By; and (3) cusp current intensity is influenced by conductivity, IMF By and their cross item.

Low‐altitude electron acceleration due to multiple flow bursts in the magnetotail

At 10:00 UT on 25 February 2008, Cluster 1 spacecraft crossed the near-midnight auroral zone, at about 2R(E) altitude, while two of the Time History of Events and Macroscale Interactions During Sub ...

Empirical Modeling of Planetary Magnetospheres in Response to Solar Wind Dynamics Using EOF Analysis and Multivariate Linear Regression

A popular approach for empirical modeling is through representing a phenomenon as a linear combination of a set of predefined basis functions, such as polynomial and harmonic functions. In comparison with predefined basis functions, the empirical orthogonal basis functions (EOFs) are constructed from the dataset according to the variance distribution, and typically summarize the data into representative features. This chapter reviews comparatively three applications of EOF analysis associated with multivariate linear regression in empirical modeling, namely in the models of the Earth’s ionospheric F 2 -layer peak, the field-aligned currents at Earth, and the induced magnetic field near Venus (He et al. Geophys. Res. Lett., 38(14): L14101, 2011; He et al. Geophys. Res. Lett., 39, 2012; He et al. 2017; He et al. J. Geophys. Res., 121(4), 3362–3380, 2016). We illustrate the physical meaning represented by the most important EOFs, detail the model constructions and methodology, and highlight the revealed main scientific results.

Multiscale estimation of the field-aligned current density

Field-aligned currents (FACs) in the magnetosphere–ionosphere (M–I) system exhibit a range of spatial and temporal scales that are linked to key dynamic coupling processes. To disentangle the scale dependence in magnetic field signatures of auroral FACs and to characterize their geometry and orientation, Bunescu et al. (2015) introduced the multiscale FAC analyzer framework based on minimum variance analysis (MVA) of magnetic time series segments. In the present report this approach is carried further to include in the analysis framework a FAC density scalogram, i.e., a multiscale representation of the FAC density time series. The new technique is validated and illustrated using synthetic data consisting of overlapping sheets of FACs at different scales. The method is applied to Swarm data showing both large-scale and quiet aurora as well as mesoscale FAC structures observed during more disturbed conditions. We show both planar and non-planar FAC structures as well as uniform and non-uniform FAC density structures. For both synthetic and Swarm data, the multiscale analysis is applied by two scale sampling schemes, namely the linear and logarithmic scanning of the FAC scale domain. The local FAC density is compared with the input FAC density for the synthetic data, whereas for the Swarm data we cross-check the results with well-established singleand dual-spacecraft techniques. All the multiscale information provides a new visualization tool for the complex FAC signatures that complements other FAC analysis tools.