Hectometric Radio Emission Research Articles

Generation of the Jovian hectometric radiation: First lessons from Juno

AbstractUsing Juno plasma and wave and magnetic observations (JADE and Waves and MAG instruments), the generation mechanism of the Jovian hectometric radio emission is analyzed. It is shown that suitable conditions for the cyclotron maser instability (CMI) are observed in the regions of the radio sources. Pronounced loss cone in the electron distributions are likely the source of free energy for the instability. The theory reveals that sufficient growth rates are obtained from the distribution functions that are measured by the JADE‐Electron instrument. The CMI would be driven by upgoing electron populations at 5–10 keV and 10–30° pitch angle, the amplified waves propagating at 82°–87° from the B field, a fraction of a percent above the gyrofrequency. Typical e‐folding times of 10−4 s are obtained, leading to an amplification path of ~1000 km. Overall, this scenario for generation of the Jovian hectometric waves differs significantly from the case of the auroral kilometric radiation at Earth.

TEMPORAL VARIATIONS OF IO'S MAGNETIC FOOTPRINT BRIGHTNESS

The brightness of Ios magnetic footprint, an indicator of electromagnetic interaction at the satellite, appears to be strongly connected to the distance from the plasma equator. As a result, the brightest footprints were detected when Io is near the interception location between the orbital plane and the plasma equator. However, volcanic activities on Io show strong correlation with the equatorward shift of Jupiters main auroral oval, consequently causing the disappearance of Ios footprint. The same conclusion was suggested via the observation of Jupiters hectometric radio emission, called HOM, which closely corresponds to Jupiters auroral activity. The plasma environment near the Jovian satellites was found to vary signicantly at dierent observational epochs. The electron density increased by approximately a factor of three from the Voyager epoch (1979) to the Galileo epoch (1995), while the electron density was found to be signicantly higher (~ 5 times) in the Cassini epoch (2001). In this current study, the magnetic footprints were clearly brighter ten years ago (from peak brightness in 1998-2001) than the footprints detected in 2007. For volcanic activities on Io in 2007, there are two clear activities in February and late May. The magnetic footprint appeared to be dimmer in March 2007, expected to be the result of volcano activities in Feb 2007. However, the magnetic footprint brightness in June appeared to be slightly brighter than the footprints observed in May. The reason could be the time delay between the brightening of the sodium nebula on approximately May 31st and, a while later, the enhancement of flux tube content peaking on approximately June 5th. On the other hand, Ios magnetic footprints were observed during June 1st - 10th when they may not yet have been affected by the increase in mass outflow due to the increase of plasma density.

Comparison between Cassini and Voyager observations of Jupiter's decametric and hectometric radio emissions

[1] In this paper, we attempt to clarify the relationship between Jovian hectometric (HOM) and non-Io-related decametric (non-Io-DAM) radio structure. For that purpose, we extend the analysis by including more data and investigating statistical properties of the Jovian DAM and HOM radio emissions based on Cassini and Voyager observations, especially below 16 MHz. We have investigated these emissions observed by the Cassini, Voyager 1, and Voyager 2 spacecraft for specific Jovigraphic latitudes in the range of −3.7°–7.3° and local times in the range of 0.76–21.4 hours. We show a statistical comparison of Cassini, Voyager 1, and Voyager 2 data for occurrence probability in Central Meridian Longitude (CML) versus Io phase and in CML versus Frequency. The main results are as follows: (1) the detailed frequency structures of non-Io-related components can be seen for different spacecraft's local time and Jovigraphic latitude, (2) the high frequency of HOM extends up to near 10 MHz, and (3) a new DAM component, named the non-Io-D, appears from 40° to 60° CML in the frequency range of 7–11 MHz. On the basis of additional information of different behaviors of non-Io-B and non-Io-A structures in longitude depending on pre- and post-encounter of Cassini data, we improved the DAM angular beaming model that shows the cone half-angle of the emitting cone decreases as a function of frequency. We conclude that the changing beaming angle is not affected by Jovigraphic latitude of the spacecraft, but rather due to different local time of the source regions.

Occurrence statistics and ray tracing study of Jovian quasiperiodic radio bursts observed from low latitudes

The occurrence characteristics of Jovian quasiperiodic (QP) bursts at a VLF range (<10 kHz) were statistically investigated using data from the Galileo spacecraft at low latitudes in the Jovian magnetosphere. The results confirmed that the occurrence of QP bursts is significantly dependent on the phase of planetary rotation rather than the central meridian longitude of the observer seen from Jupiter. It was revealed that the meridional distribution of QP bursts forms a shadow zone in the equatorial region of <30 Jovian radii from Jupiter, similar to that of hectometric radio emissions, where QP bursts are quenched. Based on the ray tracing method, we surveyed the source parameters, which can reproduce the observed shadow zone. It was suggested that the wave mode, source location, and directivity of the radio emissions are as follows: the extraordinary mode is reasonable for QP bursts observed at low latitudes, the source is located around an altitude of ∼10–20 Jovian radii above the polar region, the L value of the source field line is in a range of L > ∼20, and QP bursts could have beaming angles like “filled cone” in a restricted L value range or have a large source L value range with beaming angles like “hollow cones.” These results imply that QP bursts observed at low latitudes are generated at fRX surfaces in the polar region and propagate to the equatorial region.

A NEW MINIMALLY INVASIVE TREATMENT FOR MALE STRESS URINARY INCONTINENCE: THE ADVANCE® MALE SLING, ONE YEAR DATA FROM A MULTICENTER PROSPECTIVE STUDY

The Cassini spacecraft Ultraviolet Imaging Spectrograph (UVIS) obtained observations of Jupiter's auroral emissions in H2 band systems and H Lyman-α from day 275 of 2000 (October 1), to day 81 of 2001 (March 22). Much of the globally integrated auroral variability measured with UVIS can be explained simply in terms of the rotation of Jupiter's main auroral arcs with the planet. These arcs were also imaged by the Space Telescope Imaging Spectrograph (STIS) on Hubble Space Telescope (HST). However, several brightening events were seen by UVIS in which the global auroral output increased by a factor of 2–4. These events persisted over a number of hours and in one case can clearly be tied to a large solar coronal mass ejection event. The auroral UV emissions from these bursts also correspond to hectometric radio emission (0.5–16 MHz) increases reported by the Galileo Plasma Wave Spectrometer (PWS) and Cassini Radio and Plasma Wave Spectrometer (RPWS) experiments. In general, the hectometric radio data vary differently with longitude than the UV data because of radio wave beaming effects. The 2 largest events in the UVIS data were on 2000 day 280 (October 6) and on 2000 days 325–326 (November 20–21). The global brightening events on November 20–21 are compared with corresponding data on the interplanetary magnetic field, solar wind conditions, and energetic particle environment. ACE (Advanced Composition Explorer) solar wind data was numerically propagated from the Earth to Jupiter with an MHD code and compared to the observed event. A second class of brief auroral brightening events seen in HST (and probably UVIS) data that last for ∼2 min is associated with auroral flares inside the main auroral ovals. On January 8, 2001, from 18:45–19:35 UT UVIS H2 band emissions from the north polar region varied quasiperiodically. The varying emissions, probably due to auroral flares inside the main auroral oval, are correlated with low-frequency quasiperiodic radio bursts in the 0.6–5 kHz Galileo PWS data.

Observations of the auroral hectometric radio emission onboard the INTERBALL-1 satellite

The results of five-year (1995–2000) continuous observations of the auroral radio emission (ARE) in the hectometric wavelength range on the high-apogee INTERBALL-1 satellite are presented. Short intense bursts of the auroral hectometric radio emission (AHR) were observed at frequencies of 1463 and 1501 kHz. The bursts were observed predominantly at times when the terrestrial magnetosphere was undisturbed (in the quiet Sun period), and their number decreased rapidly with increasing solar activity. The bursts demonstrated seasonal dependence in the Northern and Southern hemispheres (dominating in the autumn-winter period). Their appearance probably depends on the observation time (UT). A qualitative explanation of the AHR peculiarities is given.

Cassini UVIS observations of Jupiter's auroral variability

The Cassini spacecraft Ultraviolet Imaging Spectrograph (UVIS) obtained observations of Jupiter's auroral emissions in H 2 band systems and H Lyman- α from day 275 of 2000 (October 1), to day 81 of 2001 (March 22). Much of the globally integrated auroral variability measured with UVIS can be explained simply in terms of the rotation of Jupiter's main auroral arcs with the planet. These arcs were also imaged by the Space Telescope Imaging Spectrograph (STIS) on Hubble Space Telescope (HST). However, several brightening events were seen by UVIS in which the global auroral output increased by a factor of 2–4. These events persisted over a number of hours and in one case can clearly be tied to a large solar coronal mass ejection event. The auroral UV emissions from these bursts also correspond to hectometric radio emission (0.5–16 MHz) increases reported by the Galileo Plasma Wave Spectrometer (PWS) and Cassini Radio and Plasma Wave Spectrometer (RPWS) experiments. In general, the hectometric radio data vary differently with longitude than the UV data because of radio wave beaming effects. The 2 largest events in the UVIS data were on 2000 day 280 (October 6) and on 2000 days 325–326 (November 20–21). The global brightening events on November 20–21 are compared with corresponding data on the interplanetary magnetic field, solar wind conditions, and energetic particle environment. ACE (Advanced Composition Explorer) solar wind data was numerically propagated from the Earth to Jupiter with an MHD code and compared to the observed event. A second class of brief auroral brightening events seen in HST (and probably UVIS) data that last for ∼2 min is associated with auroral flares inside the main auroral ovals. On January 8, 2001, from 18:45–19:35 UT UVIS H 2 band emissions from the north polar region varied quasiperiodically. The varying emissions, probably due to auroral flares inside the main auroral oval, are correlated with low-frequency quasiperiodic radio bursts in the 0.6–5 kHz Galileo PWS data.

Jovicentric latitude effect on the HOM radio emission observed by Ulysses/URAP at 5 AU from Jupiter

Abstract. During 1994 and into 1996, Ulysses was at dis-tances of 5 AU or more from Jupiter and travelling from south to north of the ecliptic plane between jovicentric latitudes -36° to 20°. Observations by the Unified Radio and Plasma Experiment (URAP) on board the Ulysses spacecraft during this period have been searched for jovian hectometric (HOM) radio events. At these distances, the HOM was only received occasionally. The signals were generally weak and much care was needed to find and to identify the events. All of the HOM events were observed when Ulysses was at jovicentric latitudes between -12.2° < DUly < 14.7°, relatively close to the plane of the jovicentric equator. Both senses of polarization were observed with left-hand (LH) predominant. The events occurred when the jovimagnetic latitude Df was between -8.5° and 14.2° and suggest that the HOM was only detectable within a beam some 23° wide, centred on about 3° jovimagnetic latitude. This is roughly consistent with previous work by Alexander et al. (1979) and by Ladreiter and Leblanc (1989, 1991), based upon observations made by Voyager and other spacecraft when these were relatively close to Jupiter. The results are consistent with an emission process due to the Cyclotron-Maser instability, as suggested by a number of scientists in the past.Key words. Magnetospheric physics (planetary magnetospheres) – Radio science (radio astronomy)

Control of Jupiter's radio emission and aurorae by the solar wind.

Radio emissions from Jupiter provided the first evidence that this giant planet has a strong magnetic field and a large magnetosphere. Jupiter also has polar aurorae, which are similar in many respects to Earth's aurorae. The radio emissions are believed to be generated along the high-latitude magnetic field lines by the same electrons that produce the aurorae, and both the radio emission in the hectometric frequency range and the aurorae vary considerably. The origin of the variability, however, has been poorly understood. Here we report simultaneous observations using the Cassini and Galileo spacecraft of hectometric radio emissions and extreme ultraviolet auroral emissions from Jupiter. Our results show that both of these emissions are triggered by interplanetary shocks propagating outward from the Sun. When such a shock arrives at Jupiter, it seems to cause a major compression and reconfiguration of the magnetosphere, which produces strong electric fields and therefore electron acceleration along the auroral field lines, similar to the processes that occur during geomagnetic storms at the Earth.

Simple ray tracing of Galileo‐observed hectometric attenuation features

Observations of persistent structural features within Jovian hectometric (HOM) radio emission have been made with the Galileo spacecraft. Two well‐defined sinusoidal‐shaped “band” features of reduced emission intensity and occurrence probability exist at all Jovian longitudes and nearly cover the entire spectrum of HOM radio emission from ∼500 kHz to 3000 kHz. These two sinusoidal lanes have a bandwidth of 200–400 kHz and are 180° out of phase with one another, suggesting that they are a result of HOM radio emission propagation processes from opposite hemispheres. These features become more apparent when presented as intensity or occurrence probability spectrograms added together over multiple Jovian rotations. Enhancements in the HOM intensity and occurrence are seen along the edges of one of the observed sinusoidal lane features which may indicate caustic surfaces due to refraction along the propagation path. We present some simple ray tracing analyses to show that refraction from density enhancements in the Io torus flux tube may explain some of the observations. Using this simple method, we approximate the density enhancements in the Io flux tube to be 100 cm−3.

Jovian hectometric beam observed by PWS and WAVES experiments on board Galileo and Wind spacecraft

We report on the jovian hectometric radio emission (HOM) observed simultaneously by PWS and WAVES experiments on board Galileo and Wind spacecraft, respectively. The positions of both spacecraft allow us to analyse the occurrence of the HOM emission in the frequency range from 300 kHz to 3 MHz . The jovian hectometric occurrence enables the study of the emission beams, in particular the geometric conditions where Galileo spacecraft is orbiting in the jovian magnetosphere and Wind satellite is close to the Earth magnetosphere. We select HOM features (arcs) observed by both spacecraft when Jupiter, Galileo and Wind spacecraft are aligned. In such a geometric configuration it is possible to compare the characteristics of the hectometric beams taking into consideration the corresponding HOM beam patterns. We show the presence of a selective effect at four specific longitudes where only small parts of this beam are observed by the Wind/WAVES experiment which may appear surprising because the global beam intensity level is nearly the same as that measured by the Galileo/PWS experiment. Those longitude values are found to be comparable to the minima and maxima of the attenuation band reported in previous studies (Gurnett et al., Geophys. Res. Lett. 25 (1988) 1841–1844). It appears that the Io satellite and its torus may have an important influence on the generation of radio emissions in HOM and also DAM domains which could explain the large frequency range of some HOM/DAM arcs which extend from 300 kHz and reach up to 40 MHz .

The Jovian HOM and bKOM radio emissions observed by Wind/WAVES and by Ulysses/URAP

During 1995 and into 1996, both the Jovian hectometric radio emission (HOM) and the Jovian broadband radio emission (bKOM) were received occasionally by the Radio and Plasma Wave Investigation (WAVES) on board the Wind spacecraft and by the Unified Radio and Plasma Experiment (URAP) on board the Ulysses spacecraft. Ulysses was then at distances of 5 AU or more from Jupiter and travelling from south to north of the ecliptic plane. At this distance from the planet, the events are generally weak and much care is needed to distinguish the HOM from weak solar type III emission and, in the Wind/WAVES observations, the bKOM from terrestrial auroral kilometric radiation (AKR). Characteristics of the emission, duration, frequency extent, structure and polarization give new information on the beaming and source location of the HOM and the bKOM. For the period studied, it was found that the bKOM events were observed by Ulysses when the spacecraft was at higher jovicentric latitudes, ( D Uly⩾15°) while HOM events were recorded when the spacecraft was at lower jovicentric latitudes, −12°⩽ D Uly⩽14°. Polarization measurements suggest that, due to beaming, LH polarized radiation from the southern Jovian hemisphere HOM source is beamed towards Ulysses at jovicentric latitudes to the north of some value between −11.7 and −5.3°. Conversely, RH polarized radiation from the northern hemisphere HOM source is beamed away from Ulysses at jovicentric latitudes to the north of some value between −11.7 and −5.3°. For the events studied, if a two-dimensional tilted dipole model is assumed, the emission cone half-angle appears to be about 44° for 940 kHz HOM emission from a source on L-shell about 10. There is no unique value for β that does not involve assumed values of L and f c (Barrow and Lecacheux, Astron. Astrophys. 301 (1995) 903.)

Cassini and Wind stereoscopic observations of Jovian nonthermal radio emissions: Measurement of beam widths

During two intervals in 1999, simultaneous observations of Jupiter's decametric and hectometric radio emissions were made with the Cassini radio and plasma wave instrument (RPWS) and the radio and plasma wave instrument (WAVES) on the Wind spacecraft in Earth orbit. During January the Jovian longitude difference between the two spacecraft was about 5°, whereas for the August–September Earth flyby of Cassini, the angle ranged from 0° to about 2.5° (the Jovicentric latitudinal difference was <0.3° during both intervals). With these separations the instantaneous widths of the walls of the hollow conical radiation beams of some of the decametric arcs were measured by cross correlating dynamic spectra. The results suggest that the typical width is approximately 1.5° ±0.5°. The conical beams seem to move at Io's revolution rate for Io‐controlled arcs. Additionally, some of the nonarc hectometric wavelength emissions show some properties of both wide and very narrow beam widths.

On the Solar Origin of Complex Type III–like Radio Bursts Observed at and below 1 MHz

By simultaneously analyzing decimetric, metric, and hectometric radio observations, we provide evidence that a class of hectometric type III-like radio events are associated with electrons accelerated during the primary flare energy release process. We do this by demonstrating that there is a good temporal correspondence between the hectometric and decimetric radio emissions that are believed to involve the acceleration of electrons in deep layers of the solar atmosphere at heights estimated to be from 10,000 to 30,000 km (0.014 to 0.043 R☉). This class of hectometric type III-like events may have simple or complex intensity-time profiles. When they have complex profiles of long duration, the decimeter emissions also have complex long-duration profiles. In this latter case they are also often associated with metric type II radio bursts and coronal mass ejections (CMEs). However, we argue on the basis of the observations that the hectometric radio emissions are not necessarily associated with the metric phenomena. Specifically, they are not likely generated by a secondary acceleration process associated with the shock generating a metric type II burst or with a shock driven ahead of the CME despite the fact that the hectometric emission time profiles would (erroneously) be morphologically classified as shock-accelerated (SA) events.

Effectiveness of near‐grazing incidence reflection in creating the rotationally modulated lanes in the Jovian hectometric radio emission spectrum

The Galileo plasma wave instrument has identified a narrow (in frequency) attenuation band in the hectometric emission that varies in frequency with system III longitude. It is possible to model this emission band assuming a high‐latitude cyclotron source region with emission that is efficiently attenuated when the ray path is nearly tangent to an L shell that is close to the Io flux tube [Gurnett et al, 1998]. The data suggest that the mechanism for attenuating the emission is very efficient, with the ratio of attenuated to unattenuated emission I/I0 < 0.02, and not a strong function of frequency. In this paper we demonstrate that incoherent scattering alone cannot explain the attenuation lane, which does not preclude coherent scattering by uncertain processes. We find rather that the source of attenuation is consistent with near‐grazing incidence reflection of emission from an L shell that is near the Io flux tube (a caustic surface).

Second harmonic hectometric radio emission at Jupiter

Galileo has been in orbit around Jupiter since December 1995. The plasma wave instrument on board the spacecraft has occasionally detected a rotationally modulated attenuation band in the hectometric (HOM) emission that most likely is due to scattering of the radiation from density fluctuations along the Io L‐shell, as reported earlier [cf. Gurnett et al., 1998]. The occurrence of the attenuation band is likely to be dependent on Io activity and the presence of density scattering centers along the Io L‐shell as well as the location of the source region. Some of the attenuation bands show clear indications of second harmonic emission. Without polarization measurements, it is difficult to place constraints on the local generation conditions based on the cyclotron maser instability, but the results imply that second harmonic emission could be present in the decametric (DAM) radiation as well. A survey of the data has revealed about 30 examples of second harmonic HOM.

Latitudinal structure within Jovian hectometric radiation

Jovian hectometric radio emission (HOM: 300–3000 kHz) has a number of persistent structural features associated with it as observed by the Voyager 1, Voyager 2, Ulysses, and Galileo spacecraft for specific jovigraphic latitudes (−4° to +7.1°) and local times (0.3 to 10.5 hours). Most notable are the presence of HOM emission between 270° and 120° central meridian longitude (CML) and the region of reduced emission intensity (a “gap”) between 120° and 270°. We displayed the Ulysses and Galileo data using time‐frequency occurrence probability spectrograms and show that the observed HOM emission features are nearly identical to those observed by the Voyager spacecraft. This implies that the HOM structure is long‐lived and fixed in its longitudinal position within the Jovian magnetosphere. HOM structure depends on small changes in the observer's jovigraphic latitude, so the different jovigraphic latitudes of the spacecraft were used to probe the HOM beam structure. Prom this analysis we found that the CML width of the main HOM gap is directly correlated to the latitude of the spacecraft. We conclude that the latitudinal thickness of the HOM beam is about 12°, extending from −5° to +7° magnetic latitude.

An unusual rotationally modulated attenuation band in the Jovian hectometric radio emission spectrum

A well‐defined attenuation band modulated by the rotation of Jupiter has been found in the spectrum of Jovian hectometric radiation using data from the Galileo plasma wave instrument. The center frequency of this band usually occurs in the frequency range from about 1 to 3 MHz and the bandwidth is about 10 to 20 percent. The center frequency varies systematically with the rotation of Jupiter and has two peaks per rotation, the first at a system III longitude of about 50°, and the second at about 185°. It is now believed that the attenuation occurs as the ray path from a high‐latitude cyclotron maser source passes approximately parallel to the magnetic field near the northern or southern edges of the Io L‐shell. The peak at 50° system III longitude is attributed to radiation from a southern hemisphere source and the peak at 185° is from a northern hemisphere source. The attenuation is thought to be caused by coherent scattering or shallow‐angle reflection from field‐aligned density irregularities near the Io L‐shell. The narrow bandwidth indicates that the density irregularities are confined to a very narrow range of L values (Δ L = 0.2 to 0.4) near the Io L‐shell.

Structure within Jovian hectometric radiation

Observations of Jovian hectometric radio emission (HOM) by the Voyager planetary radio astronomy (PRA) experiment at frequencies from 300 kHz to 1.3 MHz indicate persistent dynamic spectral features that had not been previously studied. The features of interest appear as “lanes” of decreased emission intensity within the otherwise persistent HOM. The lanes are apparent in intensity and occurrence probability spectrograms of frequency versus Jovian System III (1965) longitude. In the investigation of the morphology of these features, we use inbound and outbound Voyager 2 data at Jupiter to show that the lane occurrence and characteristics do not depend on local time over the range sampled. Occurrence probability spectrograms of frequency versus magnetic latitude are created from the portion of the data when the spacecraft was between 0° and +10° magnetic latitude. These spectrograms represent both the inbound and outbound passes and are quite similar despite the different longitude ranges. A simple extension of decametric (DAM) arc features into the HOM wavelength does not account for all the lane features, giving further evidence that HOM is an independent emission component. Polarization signatures for the data show that the polarization is predominantly right‐hand circular and that it does not reverse across the lanes, suggesting the emission is from the same hemisphere. In addition, we investigate possible effects due to solar wind variations and find that the occurrence of the lanes appears to be independent of times of low and high solar wind densities. The intensity of the HOM emission on either side of the lanes is comparable, implying that the lane is probably not a result of a gap between fundamental and second harmonic emission regions. We present these data and analyses as a morphological study to establish that the lane features are an important part of the HOM emission and should be considered in HOM emission models. At this time, no theory of the source of the lanes explains all the observed features.

Source characteristics of Jovian hectometric radio emissions

Direct confirmation that low‐frequency Jovian hectometric (HOM) radio emissions centered near 0° central meridian longitude consist of distinct, oppositely polarized northern and southern beams has been achieved using data from the Unified Radio and Plasma Wave (URAP) experiment on the Ulysses spacecraft during the Ulysses‐Jupiter encounter in early February 1992. Distinct northern and southern beams were observed in the frequency range from ∼300 kHz to 1 MHz for at least eight Jovian rotations during the Ulysses inbound pass at distances from 100 to 40 RJ. The radiation from the two magnetic hemispheres was measured from different Jovigraphic longitudes and magnetic (or centrifugal) latitudes. Observed temporal variations in the radio intensities, with time scales on the order of 30 min, may result either from longitudinal variations of the HOM sources or from longitudinal density variations in the Io plasma torus. Using the URAP direction‐finding capabilities and assuming a tilted dipole planetary magnetic field model, the three‐dimensional HOM source locations, the L shell through these source locations, and the beam opening angles were independently deduced. The HOM sources were found to originate at ∼3 RJ and on low L shells (L ∼ 4 to 6), with beam opening angles ranging from 10° to 50°.