Helmut O. Rucker

Collisional and viscous damping of MHD waves in partially ionized plasmas of the solar atmosphere

Magnetohydrodynamic (MHD) waves are widely considered as a possible source of heating for various parts of the outer solar atmosphere. Among the main energy dissipation mechanisms which convert the energy of damped MHD waves into thermal energy are collisional dissipation (resistivity) and viscosity. The presence of neutral atoms in the partially ionized plasmas of the solar photosphere, chromosphere and prominences enhances the efficiency of both these energy dissipation mechanisms. A comparative study of the efficiency of MHD wave damping in solar plasmas due to collisional and viscous energy dissipation mechanisms is presented here. The damping rates are taken from Braginskii 1965 and applied to the VAL C model of the quiet Sun (Vernazza et al. 1981). These estimations show which of the mechanisms are dominant in which regions. In general the correct description of MHD wave damping requires the consideration of all energy dissipation mechanisms via the inclusion of the appropriate terms in the generalized Ohms law, the momentum, energy and induction equations. Specific forms of the generalized Ohms Law and induction equation are presented that are suitable for regions of the solar atmosphere which are partially ionised.

Magnetohydrodynamic waves in solar partially ionized plasmas: two-fluid approach

Context. Partially ionized plasma is usually described by a single-fluid approach, where the ion-neutral collision effects are expressed by Cowling conductivity in the induction equation. However, the single-fluid approach is not valid for time-scales less than ion-neutral collision time. For these time-scales the two-fluid description is the better approximation. Aims. We aim to derive the dynamics of magnetohydrodynamic (MHD) waves in two-fluid partially ionized plasmas and to compare the results with those obtained under single-fluid description. Methods. Two-fluid equations are used, where ion-electron plasma and neutral particles are considered as separate fluids. Dispersion relations of linear waves are derived for the simplest case of homogeneous medium. Frequencies and damping rates of waves are obtained for different parameters of background plasma. Results. We found that two- and single-fluid descriptions give similar results for low-frequency waves. However, the dynamics of MHD waves in the two-fluid approach is significantly changed when the wave frequency becomes comparable with or higher than the ion-neutral collision frequency. Alfven and fast magneto-acoustic waves attain their maximum damping rate at particular frequencies (for example, the peak frequency equals 2.5 times the ion-neutral collision frequency for 50% of neutral hydrogen) in the wave spectrum. The damping rates are reduced for the higher frequency waves. The new mode of slow magneto-acoustic wave appears for higher frequency branch, which is connected to neutral hydrogen fluid. Conclusions. The single-fluid approach perfectly deals with slow processes in partially ionized plasmas, but fails for time-scales shorter than ion-neutral collision time. Therefore, the two-fluid approximation should be used for the description of relatively fast processes. Some results of the single-fluid description should be revised in future such as the damping of high-frequency Alfven waves in the solar chromosphere due to ion-neutral collisions.

Observation of similar radio signatures at Saturn and Jupiter: Implications for the magnetospheric dynamics

We report on radio signatures observed at Saturn by the Cassini RPWS experiment which are strikingly similar to the Jovian “energetic events” observed by Galileo. They consist of sudden intensifications of the auroral radio emission (SKR) followed by the detection of a periodic narrowband radiation which most likely originates from Saturns plasma disk. About ten “events” have been observed in 2006, showing on average temporal scales ∼3 times longer than their Jovian counterparts. We analyze the conditions of generation and the visibility of the narrowband radiation and conclude that the Kronian “events” are most likely associated with plasma evacuation from the disk. These observations provide new insights on the role of internal energy releases in Saturns magnetosphere, known from other observations to be mainly driven by the solar wind.

Cassini model rheometry

Rheometry serves as a method for the determination of effective length vectors of short antennas by means of electrolytic tank measurements. This paper reports on the application of rheometry to the three linear monopoles mounted for the purposes of the Radio and Plasma Wave Science Experiment on the Cassini spacecraft, which will fly to planet Saturn. The voltage signals induced by incoming waves from the Saturnian radio emissions will be recorded for further evaluation. By direction-finding techniques one will trace back from the collected data to the source regions of the received radio waves and determine the wave polarization. An accurate direction finding is only possible if the effective length vectors of the antennas, which are affected by the spacecraft body, are known to a certain degree of accuracy. It is investigated how rheometry enables the determination of the effective length vectors with the help of a scale model. After a detailed discussion of the fundamentals of rheometry, the application of rheometry to the Cassini scale model is described. The results of the measurements are graphically depicted and discussed with the requirements for direction finding taken into consideration. Finally, an overview of the inflight antenna calibration is given, which will be possible by utilizing the strong Jovian radio emissions during Cassinis Jupiter flyby.

Analysis of electromagnetic wave direction finding performed by spaceborne antennas using singular-value decomposition techniques

By using two rotating noncollinear antennas or three spatially fixed noncoplanar antennas on a spacecraft, full information on the polarization and the direction of arrival of an electromagnetic wave can be obtained by measuring the voltages created by the electric field of the incident wave. The physical parameters (polarization and direction of arrival) of the incoming wave are related to the received voltages on the antenna system by the so-called direction-finding equations. Since the used antennas are generally of small directivity (electrically short monopoles or dipoles), the resulting system of equations is numerically close to singular, and generally no unique solution can be obtained for the physical parameters of the wave throughout the inversion process. However, there exists a very powerful tool for dealing with sets of equations that are singular or close to singular, known as singular-value decomposition (SVD), which precisely focuses the problem. For illustration, this paper analyzes the direction-finding equations for the Radio and Plasma Wave Science (RPWS) experiment on the Cassini spacecraft by using SVD techniques. It also compares the expected performances of RPWS with those of the Ulysses Unified Radio and Plasma Wave (URAP) experiment achieved at Jupiter for the kilometer and hectometer emissions. The RPWS experiment on Cassini, which will be launched in 1997, is supposed to observe wave phenomena between a few hundred Hertz and 16 MHz in the Saturnian magnetosphere.

LOFAR tied-array imaging of Type III solar radio bursts

The Sun is an active source of radio emission which is often associated with energetic phenomena such as solar flares and coronal mass ejections (CMEs). At low radio frequencies (<100 MHz), the Sun has not been imaged extensively because of the instrumental limitations of previous radio telescopes. Here, the combined high spatial, spectral and temporal resolution of the Low Frequency Array (LOFAR) was used to study solar Type III radio bursts at 30-90 MHz and their association with CMEs. The Sun was imaged with 126 simultaneous tied-array beams within 5 solar radii of the solar centre. This method offers benefits over standard interferometric imaging since each beam produces high temporal (83 ms) and spectral resolution (12.5 kHz) dynamic spectra at an array of spatial locations centred on the Sun. LOFARs standard interferometric output is currently limited to one image per second. Over a period of 30 minutes, multiple Type III radio bursts were observed, a number of which were found to be located at high altitudes (4 solar radii from the solar center at 30 MHz) and to have non-radial trajectories. These bursts occurred at altitudes in excess of values predicted by 1D radial electron density models. The non-radial high altitude Type III bursts were found to be associated with the expanding flank of a CME. The CME may have compressed neighbouring streamer plasma producing larger electron densities at high altitudes, while the non-radial burst trajectories can be explained by the deflection of radial magnetic fields as the CME expanded in the low corona.

Lightning activity on Titan: can Cassini detect it?

Although no lightning discharges were observed during the Voyager 1 9yby of Titan, this lack of evidence does not rule out the existence of lightning phenomena which could be detected by the Radio and Plasma Wave Science (RPWS) instrument on board of the Cassini spacecraft. The existence of lightning or other electromagnetic discharges has been suggested to explain the formation of hydrocarbons and nitriles in the context of Titan’s complex organic chemistry. Although thunderclouds may be a rare phenomenon in Titan’s lower atmosphere, recent investigations show that such clouds may cause temporary maximum electrical =elds in the order of 2× 10 6 Vm −1 su>cient to initiate 20 km long Earth-like cloud-to-ground type 2 lightning strokes. Since such clouds are likely to be correlated with strongconvection near the subsolar point, we expect possible lig htning9ashes to occur only on the dayside. Recent telescopic infrared observations have detected localized, high, short-lived clouds on Titan. We have calculated the 9ash characteristics, frequency spectrum, maximum spectral energy and the electromagnetic energy radiated into the troposphere by using a wave guide model of lightning currents for Titan’s lightning strokes. Our study indicates that cloud-to-ground lightning strokes on Titan would be comparable with so-called type 2 lightning strokes on Earth. Their total radiated energy to the far =eld could be about 130 kJ and their maximum energy at a frequency of about 4 kHz. In order to estimate the capability of the Cassini=RPWS instrument to detect lightning discharges during several close Titan 9ybys, we distinguish the atmospheric regions, where the propagation of electromagnetic waves is unperturbed or where it is impossible. We found that the Cassini=RPWS instrument should be able to detect electromagnetic signals generated from a representative cloud-to-ground lightning stroke in Titan’s lower atmosphere in a frequency range above 500 kHz or 1 MHz up to 200 Titan radii away. We suggest that for the search of lightning signals the RPWS high-frequency receiver HF2 with its H2-1E or H2-1E=F receiver modes and low integration times D t of 10 or 20 ms should be chosen. Since the lightning 9ash rate might be low (i1 9ash per hour) it is important to have longobservation times. An analysis of all Cassini trajectories of Titan close 9ybys shows that the spacecraft would have the opportunity to observe Titan’s dayside within 100 Titan radii duringnearly all 9ybys. This time should be longenoug h, even if the lightning 9ash rate is low. c

Rheometry of multi-port spaceborne antennas including mutual antenna capacitances and application to STEREO/WAVES

Electrolytic tank measurements were applied often to determine the antenna properties of spaceborne monopole antennas. This technique, called rheometry in the present context, yields the effective length vectors of antennas for the quasi-static frequency range. When receivers or preamplifiers are connected to the antennas, their input impedances and the capacitances of the cables change the effective length vectors. In the evaluation of former rheometry measurements this effect has been taken into account for each antenna individually, tacitly neglecting the mutual antenna capacitances. Our analysis shows that this neglect can cause errors in the measured effective axes (directions of effective length vectors) of up to about 10°. However, in order to apply direction finding techniques with a triaxial antenna system an accuracy of about 2° is required. We therefore extend rheometry to the measurement of the complete antenna capacitance matrix. An application of the presented technique to the WAVES antenna system onboard the STEREO spacecraft verifies that the mutual capacitances are crucial in this context. With the extended rheometry technique the antenna transfer and capacitance matrices can be completely determined, thereby yielding a full representation of the reception properties of the antenna system as part of an electronic circuit.

Damping of Alfven waves in solar partially ionized plasmas: effect of neutral helium in multi-fluid approach

Context. Chromospheric and prominence plasmas contain neutral atoms, which may change the plasma dynamics through collision with ions. Most of the atoms are neutral hydrogen, but a significant amount of neutral helium may also be present in the plasma with a particular temperature. Damping of MHD waves due to ion collision with neutral hydrogen has been studied well, but the effects of neutral helium are largely unknown. Aims. We aim to study the effect of neutral helium in the damping of Alfven waves in solar, partially ionized plasmas. Methods. We consider a three-fluid magnetohydrodynamic (MHD) approximation, where one component is electron-proton-singly ionized helium and the other two components are the neutral hydrogen and neutral helium atoms. We derive the dispersion relation of linear Alfven waves in isothermal and homogeneous plasma. Then we solve the dispersion relation and derive the damping rates of Alfven waves for different plasma parameters. Results. The presence of neutral helium significantly enhances the damping of Alfven waves compared to the damping due to neutral hydrogen at certain values of plasma temperature (10000−40000 K) and ionization. Damping rates have a peak near the ion-neutral collision frequency, but decrease for the higher part of the wave spectrum. Conclusions. The collision of ions with neutral helium atoms can be important for the damping of Alfven waves in chromospheric spicules and in prominence-corona transition regions.

LOFAR tied-array imaging and spectroscopy of solar S bursts

Context. The Sun is an active source of radio emission that is often associated with energetic phenomena ranging from nanoflares to coronal mass ejections (CMEs). At low radio frequencies (<100 MHz), numerous millisecond duration radio bursts have been reported, such as radio spikes or solar S bursts (where S stands for short). To date, these have neither been studied extensively nor imaged because of the instrumental limitations of previous radio telescopes. Aims. Here, Low Frequency Array (LOFAR) observations were used to study the spectral and spatial characteristics of a multitude of S bursts, as well as their origin and possible emission mechanisms. Methods. We used 170 simultaneous tied-array beams for spectroscopy and imaging of S bursts. Since S bursts have short timescales and fine frequency structures, high cadence (~50 ms) tied-array images were used instead of standard interferometric imaging, that is currently limited to one image per second. Results. On 9 July 2013, over 3000 S bursts were observed over a time period of ~8 hours. S bursts were found to appear as groups of short-lived (<1 s) and narrow-bandwidth (~2.5 MHz) features, the majority drifting at ~3.5 MHz/s and a wide range of circular polarisation degrees (2-8 times more polarised than the accompanying Type III bursts). Extrapolation of the photospheric magnetic field using the potential field source surface (PFSS) model suggests that S bursts are associated with a trans-equatorial loop system that connects an active region in the southern hemisphere to a bipolar region of plage in the northern hemisphere. Conclusions. We have identified polarised, short-lived solar radio bursts that have never been imaged before. They are observed at a height and frequency range where plasma emission is the dominant emission mechanism, however they possess some of the characteristics of electron-cyclotron maser emission.