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Search Results: 1 - 10 of 330613 matches for " H. Lühr "
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The efficiency of mechanisms driving Subauroral Polarization Streams (SAPS)
H. Wang,H. Lühr
Annales Geophysicae (ANGEO) , 2011,
Abstract: We have investigated the seasonal and diurnal variation of SAPS (Subauroral Polarization Streams) occurrence based on 3663 SAPS events identified in DMSP ion drift observations in the Northern Hemisphere during July 2001 and June 2003. Their relationships with high latitude convection electric field, substorm, and ionospheric conductivity have been addressed. SAPS occurrences show a clear seasonal and diurnal variation with the occurrence rates varying by a factor of 5. It is found that the convection electric field might play a dominant role in association with SAPS occurrence. Peak convection electric fields mark the occurrence maximum of SAPS. Substorm might play a secondary role related to SAPS occurrence. It account for the secondary maximum in SAPS occurrence rate during December solstice. Our work demonstrates that the substorm induced electric field can develop SAPS during relatively low global convection. Somewhat low fluxtube-integrated conductivity is favorable for SAPS to develop. Another topic is the temporal relationship between SAPS and substorm phases. SAPS can occur at substorm onset, substorm expansion and recovery phases. Most probably SAPS tend to occur 60 min/45 min after substorm onset during quiet/more disturbed geomagnetic activity, respectively. This indicates that enhanced global convection helps SAPS to develop quicker during substorms. The peak plasma velocity of SAPS is increased on average only by 5–10 % by the substorm process.
New plasmapause model derived from CHAMP field-aligned current signatures
B. Heilig,H. Lühr
Annales Geophysicae (ANGEO) , 2013,
Abstract: We introduce a new model for the plasmapause location in the equatorial plane. The determination of the L-shell bounding the plasmasphere is based on magnetic field observations made by the CHAMP satellite in the topside ionosphere. Related signals are medium-scale field-aligned currents (MSFAC) (some 10 km scale size). The mid-latitude boundary of these MSFACs is used for determining the plasmapause. We are presenting a procedure for detecting the MSFAC boundary. Reliable L-values are obtained on the night side, whenever the solar zenith angle is below 90°. This means, the boundary is not determined well in the 08:00 to 16:00 magnetic local time (MLT) sector. The radial distance of the boundary is closely controlled by the magnetic activity index Kp. Over the Kp range 0 to 9, the L-value varies from 6 to 2 RE. Conversely, the dependence on solar flux is insignificant. For a fixed Kp level, the obtained L-values of the boundary form a ring on an MLT dial plot with a centre somewhat offset from the geomagnetic pole. This Kp and local time dependent feature is used for predicting the location of the MSFAC boundary at all MLTs based on a single L-value determination by CHAMP. We compared the location of the MSFAC boundary during the years 2001–2002 with the L-value of the plasmapause, determined from in situ observations by the IMAGE spacecraft. The mean difference in radial distance is within a 1 RE range for all local times and Kp values. The plasmapause is generally found earthward of the FAC boundary, except for the duskside. By considering this systematic displacement and by taking into account the diurnal variation and Kp-dependence of the residuals, we are able to construct an empirical model of the plasmapause location that is based on MSFAC measurements from CHAMP. Our new model PPCH-2012 agrees with IMAGE in situ observations within a standard deviation of 0.79 RE.
Search for magnetically quiet CHAMP polar passes and the characteristics of ionospheric currents during the dark season
P. Ritter ,H. Lühr
Annales Geophysicae (ANGEO) , 2006,
Abstract: The magnetic activity at auroral latitudes is strongly dependent on season. During the dark season, when the solar zenith angle in the polar region is larger than 100° at all local times, the ionospheric conductivity is much reduced, and generally low activity is encountered. These time intervals are of special interest for the main field modelling, because then the geomagnetic field readings, in particular the field magnitude, are only slightly affected by ionospheric currents. Based on CHAMP data, this study examines how these quiet periods are reflected in the different magnetic field components. The peak FAC density is used as a possible proxy for the deviation of the total field. As a second option, the transverse field component, which is aligned with the auroral oval, is investigated, because it presents a measure for the FAC total current. Correlation analyses with the scalar residuals are performed and both proxies are tested for their suitability of predicting the intensity of the auroral electrojet during the dark polar seasons. The indicators based on the local FAC strength or on the amplitude of the transverse component show a reasonable correlation with the electrojet intensity for these periods, but fail when limited to small amplitudes. The predictability improves considerably if the time sector is limited to dayside hours (08:00–16:00 MLT). As the activity at high latitudes is strongly controlled by the solar wind input, we also consider IMF quantities which may support very quiet conditions. Correlations of the magnetic field scalar residuals with the merging electric field are strongest if only passes in the dayside sector are considered. Best selection results for quiet passes are obtained by combining four conditions: dark season, small average merging electric field, Em<0.8 mV/m, absence of peak values of Em>1.2 mV/m during a time interval of 40 min centred at the polar crossing, and limitation to the dayside sector (08:00–16:00 MLT). The set of quiet polar passes identified by these criteria may be used beneficially in crustal field modelling of the polar regions.
Near-Earth magnetic signature of magnetospheric substorms and an improved substorm current model
P. Ritter ,H. Lühr
Annales Geophysicae (ANGEO) , 2008,
Abstract: Based on a comprehensive catalogue with more than 4000 magnetospheric substorm entries from the years 2000–2005, the spatial distribution of the substorm-related magnetic signatures at mid and low latitudes around local midnight was investigated. Superposed epoch analysis of a larger number of recent observatory data from mid and low latitudes revealed a field strength increase that is consistent with the results of earlier studies. For the first time, the magnetic signature of the substorm current wedge formation is studied also in near-Earth satellite data from CHAMP. The average maximal deflection measured on board the satellite is smaller by a factor of 2 than that determined from ground observations. The recurrence frequency of substorms as well as the amplitude of their magnetic signature depends strongly on the prevailing magnetic activity. The observed average substorm-related magnetic field signatures cannot be described adequately by a simple current wedge model. A satisfactory agreement between model results and observations at satellite height and on ground can be achieved only if the current reconfiguration scenario combines four elements: (1) the gradual decrease of the tail lobe field, (2) the re-routing of a part of the cross-tail current through the ionosphere, (3) eastward ionospheric currents at low and mid latitudes driven by Region-2 field-aligned currents, and (4) a partial ring current connected to these Region-2 FACs.
Climatology of the cusp-related thermospheric mass density anomaly, as derived from CHAMP observations
S. Rentz ,H. Lühr
Annales Geophysicae (ANGEO) , 2008,
Abstract: We report on the thermospheric mass density anomaly in the vicinity of the ionospheric cusp. A systematic survey of the anomalies is presented, based on a statistical analysis of 4 years of data (2002–2005) obtained by the accelerometer onboard CHAMP. The anomalies are detected during all years and seasons in both hemispheres but with stronger signatures in the Northern Hemisphere. For the same geophysical conditions, solar flux and geomagnetic activity the anomalies in the north are larger by a factor of about 1.35. Over the course of the survey period the amplitude decreases by more than a factor of 5 while the level of solar flux reduces by a factor of 2. The anomaly strength also depends on the solar wind input. The merging electric field, Emerg, is generally enhanced for about an hour before the anomaly detection. There is a quadratic response of the anomaly amplitude to Emerg. For geophysical conditions of P10.7<150 and Emerg<1 mV/m hardly any events are detected. Their amplitudes are found to be controlled by an additive effect of P10.7 and Emerg, where the weight of Emerg, in mV/m, is by about 50 times higher than that of the solar flux level. The solar zenith angle and the influence of particle precipitation are found to play a minor role as a controlling parameter of seasonal variation. The well-known annual variation of the thermospheric density with a minimum around June also influences the formation of the cusp anomalies. This leads to a clear hemispheric asymmetry with very weak anomalies in the south during June solstice, which is supposed to be a combined effect of the minimum in annual variation and the seasonal decrease of solar insolation in this region.
Nonmigrating tidal signals in the upper thermospheric zonal wind at equatorial latitudes as observed by CHAMP
K. H usler ,H. Lühr
Annales Geophysicae (ANGEO) , 2009,
Abstract: The accelerometer onboard CHAMP enables us to derive the thermospheric zonal wind at orbit altitudes (~400 km). Numerous equatorial overflights (~45 250) are used to investigate the influence of nonmigrating tides on the thermospheric zonal wind. In a previous study a so called "wave-4" longitudinally signal observed in the satellite frame was identified in the zonal wind residuals during equinox. Using four years of data (2002–2005), we determine the annual variation of this prominent feature which is strongest during the months of July through October and has a smaller second maximum during March/April. Due to the large data set we were able to separate the observed wavenumbers into the tidal components. Thereby, we can identify the eastward propagating diurnal tide with zonal wavenumber s=3 (DE3) as the prime cause for the observed wave-4 pattern in the zonal wind. Analyzing the zonal wind along the geographic and the dip equator revealed that the largest amplitudes of DE3 are found along the dip equator. Besides DE3 we present the full spectrum of nonmigrating tides in the upper thermosphere.
Magnetic properties of CHAMP and their effects on in-orbit calibration
H. Lühr, F. Yin,R. Bock
Journal of Sensors and Sensor Systems (JSSS) , 2013, DOI: 10.5194/jsss-2-9-2013
Abstract: CHAMP has so far been the most successful magnetic field mission. For achieving that, special effort had to be invested in building a magnetically clean spacecraft. The magnetic moment of the spacecraft is about 1 Am2. In this article we introduce a new method that allows the determination of the magnetic moment during the mission from the average current strength of the magneto-torquers. In order to achieve precise field vector data, the readings of the fluxgate magnetometer are routinely calibrated against the absolute Overhauser measurements. A reanalysis of all the magnetic field data is performed which takes also into account small disturbances from the power system. Uncertainties of the final magnetic field data are estimated to be of the order of 0.1 nT.
CHAMP observation of intense kilometer-scale field-aligned currents, evidence for an ionospheric Alfvén resonator
M. Rother, K. Schlegel,H. Lühr
Annales Geophysicae (ANGEO) , 2007,
Abstract: Bursts of very intense kilometer-scale field-aligned currents (KSFACs) are observed quite frequently by the CHAMP satellite when passing through the auroral region. In extreme cases estimated current densities exceed 3 mA/m2. Typical scale sizes of these KSFACs are 1 km. The low-Earth, polar orbiting satellite CHAMP allows one to assess KSFACs down to scales of a couple of 100 m based on its high-precision magnetic field vector data sampled at 50 Hz. Using data from 5 years (2001–2005) details of these currents can be investigated. In our statistical study we find that most of the KSFAC bursts and the strongest events are encountered in the cusp/cleft region. Significantly fewer events are found on the nightside. The affected region is typically 15°–20° wide in latitude. There seems to be some dependence of the current intensity on the level of magnetic activity, Kp. On the other hand, no dependence has been found on sunspot number, the solar flux level, F10.7 or the solar zenith angle. The latitude, at which KSFAC bursts are encountered, expands equatorward with increasing Kp. This trend follows well the auroral oval expansion during enhanced magnetic activity. These KSFACs are generally accompanying large-scale FAC sheets, and they are predominantly associated with Region 1 currents. We propose an explanation of the KSFACs in terms of Alfvén waves trapped in a ionospheric resonator, which is initiated when the convection electric field or current strength surpasses a critical value. Many properties of such a resonator are in agreement with our KSFAC results.
Substorm-related thermospheric density and wind disturbances derived from CHAMP observations
P. Ritter, H. Lühr,E. Doornbos
Annales Geophysicae (ANGEO) , 2010,
Abstract: The input of energy and momentum from the magnetosphere is most efficiently coupled into the high latitude ionosphere-thermosphere. The phenomenon we are focusing on here is the magnetospheric substorm. This paper presents substorm related observations of the thermosphere derived from the CHAMP satellite. With its sensitive accelerometer the satellite can measure the air density and zonal winds. Based on a large number of substorm events the average high and low latitude thermospheric response to substorm onsets was deduced. During magnetic substorms the thermospheric density is enhanced first at high latitudes. Then the disturbance travels at an average speed of 650 m/s to lower latitudes, and 3–4 h later the bulge reaches the equator on the night side. Under the influence of the Coriolis force the travelling atmospheric disturbance (TAD) is deflected westward. In accordance with present-day atmospheric models the disturbance zonal wind velocities during substorms are close to zero near the equator before midnight and attain moderate westward velocities after midnight. In general, the wind system is only weakly perturbed (Δvy<20 m/s) by substorms.
The relationship of thermospheric density anomaly with electron temperature, small-scale FAC, and ion up-flow in the cusp region, as observed by CHAMP and DMSP satellites
G. N. Kervalishvili ,H. Lühr
Annales Geophysicae (ANGEO) , 2013,
Abstract: We present in a statistical study a comparison of thermospheric mass density enhancements (ρrel) with electron temperature (Te), small-scale field-aligned currents (SSFACs), and vertical ion velocity (Vz) at high latitudes around noon magnetic local time (MLT). Satellite data from CHAMP (CHAllenging Minisatellite Payload) and DMSP (Defense Meteorological Satellite Program) sampling the Northern Hemisphere during the years 2002–2005 are used. In a first step we investigate the distribution of the measured quantities in a magnetic latitude (MLat) versus MLT frame. All considered variables exhibit prominent peak amplitudes in the cusp region. A superposed epoch analysis was performed to examine causal relationship between the quantities. The occurrence of a thermospheric relative mass density anomaly, ρrel >1.2, in the cusp region is defining an event. The location of the density peak is taken as a reference latitude (Δ MLat = 0°). Interestingly, all the considered quantities, SSFACs, Te, and Vz are co-located with the density anomaly. The amplitudes of the peaks exhibit different characters of seasonal variation. The average relative density enhancement of the more prominent density peaks considered in this study amounts to 1.33 during all seasons. As expected, SSFACs are largest in summer with average amplitudes equal to 2.56 μA m 2, decaying to 2.00 μA m 2 in winter. The event related enhancements of Te and Vz are both largest in winter (Δ Te =730 K, Vz =136 m s 1) and smallest in summer (Δ Te = 377 K, Vz = 57 m s 1. Based on the similarity of the seasonal behaviour we suggest a close relationship between these two quantities. A correlation analysis supports a linear relation with a high coefficient greater than or equal to 0.93, irrespective of season. Our preferred explanation is that dayside reconnection fuels Joule heating of the thermosphere causing air upwelling and at the same time heating of the electron gas that pulls up ions along affected flux tubes.
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