Jovian Magnetic Field Research Articles

Europa's far ultraviolet oxygen aurora from a comprehensive set of HST observations

AbstractWe analyze a large set of far ultraviolet oxygen aurora images of Europa's atmosphere taken by Hubble's Space Telescope Imaging Spectrograph (HST/STIS) in 1999 and on 19 occasions between 2012 and 2015. We find that both brightness and aurora morphology undergo systematic variations correlated to the periodically changing plasma environment. The time variable morphology seems to be strongly affected by Europa's interaction with the magnetospheric plasma. The brightest emissions are often found in the polar region where the ambient Jovian magnetic field line is normal to Europa's disk. Near the equator, where bright spots are found at Io, Europa's aurora is faint suggesting a general difference in how the plasma interaction shapes the aurora at Io and Europa. The dusk side is consistently brighter than the dawnside with only few exceptions, which cannot be readily explained by obvious plasma physical or known atmospheric effects. Brightness ratios of the near‐surface OI] 1356 Å to OI 1304 Å emissions between 1.5 and 2.8 with a mean ratio of 2.0 are measured, confirming that Europa's bound atmosphere is dominated by O2. The 1356/1304 ratio decreases with increasing altitude in agreement with a more extended atomic O corona, but O2 prevails at least up to altitudes of ∼900 km. Differing 1356/1304 line ratios on the plasma upstream and downstream hemispheres are explained by a differing O mixing ratio in the near‐surface O2 atmosphere of ∼5% (upstream) and 1% (downstream), respectively. During several eclipse observations, the aurora does not reveal any signs of systematic changes compared to the sunlit images suggesting no or only weak influence of sunlight on the aurora and an optically thin atmosphere.

Development of a Magnetic Loop Antenna for the Detection of Jovian Radiowaves at 20.1 MHz

Radio waves coming from the planet Jupiter are electromagnetic phenomena generated by synchrotron emission mechanism, due to the interaction of Jovian magnetic field with charged particles produced by the satellite Io. These radio waves fall directly upon the ionosphere affecting earth’s radio communications. At the Astronomical Observatory of the University of Technology of Pereira (OAUTP) the research group in Astro-engineering Alfa Orion, developed a magnetic loop antenna designed to resonate at 20.1 MHz, which coupled with the receptor and the software from NASA’s Radio Jove project enabled the observation of radio sources A, B, C, IoA, IoB and IoC from Jupiter, allowing the construction of a database for the study of the Jovian activity in radio waves.

Aurora at high latitudes of Ganymede

The aurora on Ganymede results from the interaction of Jovian plasma disk electrons with Ganymede’s tenuous atmosphere. At the same time, the electrons surrounding Ganymede have energy insufficient for the excitation of emission in the observed lines. They can acquire the necessary acceleration through various processes, such as reconnection of Ganymede’s intrinsic magnetic field with the Jovian magnetic field in the upstream and downstream regions. The generation mechanism of a field-aligned potential drop on auroral field lines is considered. The lack of charge carriers required for the generation of the necessary current closing the Alfven-wing field-aligned currents is shown to be responsible for the formation of an accelerating potential and the acceleration of thermal magnetospheric electrons. The total current in the circuit is determined by the calculated cross-field potential drop applied to Ganymede and by the Pedersen conductivity of Ganymede’s ionosphere.

Radar signal propagation through the ionosphere of Europa

We review the current state of knowledge of the Europan plasma environment, its effects on radio wave propagation, and its impact on the performance and design of future radar sounders for the exploration of Europa׳s ice crust. The Europan ionosphere is produced in two independently-rotating hemispheres by photo-ionization of the neutral exosphere and interaction with the Io plasma torus, respectively. This combination is responsible for temporal and longitudinal ionospheric heterogeneities not well constrained by observations. When Europa׳s ionosphere is active, the maximum cut-off frequency is 1MHz at the surface. The main impacts on radar signal propagation are dispersive phase shift and Faraday rotation, both a function of the total electron content (up to 4×1015m−2) and the Jovian magnetic field strength at Europa (~420nT). The severity of these impacts decrease with increasing center frequency and increase with altitude, latitude, and bandwidth. The 9MHz channels on the Radar for Icy Moons Exploration (RIME) and proposed Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) will be sensitive to the Europan ionosphere. For these or similar radar sounders, the ionospheric signal distortion from dispersive phase shift can be corrected with existing techniques, which would also enable the estimation of the total electron content below the spacecraft. At 9MHz, the Faraday fading is not expected to exceed 6dB under the worst conditions. At lower frequencies, any active or passive radio probing of the ice shell exploration would be limited to frequencies above 1–8MHz (depending on survey configuration) below which Faraday rotation angle would lead to signal fading and detection ambiguity. Radar instruments could be sensitive to neutrals and electrons added in the exosphere from any plume activity if present.

Lorentz force time-optimal transfer trajectory design in Jovian magnetic field

In this paper, the Lorentz force in Jupiter’s magnetic field is used to design the transfer trajectory between Galileo moons’ Lagrange points. The equatorial orbits of charged spacecraft in three-body axis-aligned nontilted-dipole magnetic field model are analyzed and the results show that the libration point L1 and L2 become nearer or further away from Europa with the variable size and polarity of the charge. The bang–bang charge control with the variable size and polarity of the charge can be used to change orbit’s direction and shape. Analytical and numerical iteration methods give the fast and accurate bang–bang charge control to send the spacecraft to the L2 point of Europa, respectively. Finally, two numerical methods, the indirect and direct methods, give the time-optimal charge controls which are similar to but different from the semi-revolution variable polarity control maneuver. The optimal results achieve the final position and the flight angle at the same time, and need less time and less magnitude of charge compared with the numerical iteration method.

Remote sensing of the Io torus plasma ribbon using natural radio occultation of the Jovian radio emissions

Abstract. We study the Jovian hectometric (HOM) emissions recorded by the RPWS (Radio and Plasma Wave Science) experiment onboard the Cassini spacecraft during its Jupiter flyby. We analyze the attenuation band associated with the intensity extinction of HOM radiation. This phenomenon is interpreted as a refraction effect of the Jovian hectometric emission inside the Io plasma torus. This attenuation band was regularly observed during periods of more than 5 months, from the beginning of October 2000 to the end of March 2001. We estimate for this period the variation of the electron density versus the central meridian longitude (CML). We find a clear local time dependence. Hence the electron density was not higher than 5.0 × 104 cm−3 during 2 months, when the spacecraft approached the planet on the dayside. In the late afternoon and evening sectors, the electron density increases to 1.5 × 105 cm−3 and reach a higher value at some specific occasions. Additionally, we show that ultraviolet and hectometric wavelength observations have common features related to the morphology of the Io plasma torus. The maxima of enhancements/attenuations of UV/HOM observations occur close to the longitudes of the tip of the magnetic dipole in the southern hemisphere (20° CML) and in the northern hemisphere (200° CML), respectively. This is a significant indication about the importance of the Jovian magnetic field as a physical parameter in the coupling process between Jupiter and the Io satellite.

Explaining Jupiter's magnetic field and equatorial jet dynamics

AbstractSpacecraft data reveal a very Earth‐like Jovian magnetic field. This is surprising since numerical simulations have shown that the vastly different interiors of terrestrial and gas planets can strongly affect the internal dynamo process. Here we present the first numerical dynamo that manages to match the structure and strength of the observed magnetic field by embracing the newest models for Jupiter's interior. Simulated dynamo action primarily occurs in the deep high electrical conductivity region, while zonal flows are dynamically constrained to a strong equatorial jet in the outer envelope of low conductivity. Our model reproduces the structure and strength of the observed global magnetic field and predicts that secondary dynamo action associated to the equatorial jet produces banded magnetic features likely observable by the Juno mission. Secular variation in our model scales to about 2000 nT per year and should also be observable during the 1 year nominal mission duration.

A mathematical model of the sunspot cycle for the past 1000 yr

Using many features of Ian Wilson's Tidal Torque theory, a mathematical model of the sunspot cycle has been created that reproduces changing sunspot cycle lengths and has an 85 % correlation with the sunspot numbers from 1749 to 2013. The model makes a reasonable representation of the sunspot cycle for the past 1000 yr, placing all the solar minimums in their right time periods. More importantly, I believe the model can be used to forecast future solar cycles quantitatively for 30 yr and directionally for 100 yr. The forecast is for a solar minimum and quiet Sun for the next 30 to 100 yr. The model is a slowly changing chaotic system with patterns that are never repeated in exactly the same way. Inferences as to the causes of the sunspot cycle patterns can be made by looking at the model's terms and relating them to aspects of the Tidal Torque theory and, possibly, Jovian magnetic field interactions.

How could the Io footprint disappear?

The interaction of Io with the Jovian magnetosphere is the best known – and the most intense – case of satellite–magnetosphere interaction. The interaction involves a power of more than a TeraWatt, a few percent of which are transferred to electrons. These electrons precipitate in the Jovian ionosphere where they light-up bright auroral emissions (several 100's kR to a few 1000's kR). The brightness of the Io-controlled UV auroras is known to vary, due to the Jovian magnetic field tilt, which induces longitudinal variations, and due to the Io torus ever changing parameters (possibly due to Io's volcanic activity), which induce a temporal variation. As Io-controlled UV auroras have been monitored for a long time, the variation of their brightness is well-documented, and the typical amplitude of these variations has been established. However, on June 7, 2007 an unusual event occurred in the plasma torus surrounding Io, which triggered its own UV emissions on Jupiter in a region mapping to Io's orbit. When Io reached that region, Io's auroral footprint disappeared, its brightness dimming by at least a factor of three to be below the background aurora brightness. Both the auroral event at such a low latitude and the Io footprint disappearance are events that have never been observed before and should be quite rare. However, the question of how the bright Io footprint becomes that weak remains.From a theoretical point-of-view, the Io–Jupiter interaction has been widely studied. In the 80's, it was shown that Alfvén waves are radiated from Io, carrying currents to Jupiter. In the late-2000's, studies showed that dispersive Alfvén waves were likely to cause the acceleration of the electrons powering the auroral emissions, although Io-scale Alfvén waves should be non-dispersive. More recently, a model was built which permits one to compute the ratio between dispersive and non-dispersive waves in the auroral region for satellite–magnetosphere interactions, and thus the brightness of the related aurorae. We use this model to investigate which variation of the interaction parameters could lead to the Io footprint disappearance.

Aurora on Ganymede

We present four sets of ultraviolet images of Ganymede acquired with the Hubble Space Telescope (HST) from 1998 to 2007, all of which show auroral emission from electron excited atomic oxygen. The three different hemispheres of Ganymede captured in the observations show strikingly different emission morphologies. Ultraviolet emission at 1356 Å is brightest at relatively high latitude on the orbital trailing (upstream plasma) hemisphere and in an auroral oval that extends to as low as ~10°N latitude on the orbital leading (downstream plasma) hemisphere. Two sets of images of the Jupiter‐facing hemisphere acquired at nearly the same sub‐Earth longitude but separated by ~4 years show very similar emission morphology that is consistent with the pattern of emission seen in the upstream and downstream images: the emission is at high latitude in the upstream quadrant and at low latitude in the downstream quadrant. This implies that the large‐scale, nominal “auroral oval” on Ganymede is apparently quite stable with time, despite significant brightness fluctuations within the overall stable pattern during the 10–30 min time scale between individual images. The overall emission morphology appears to be driven primarily by the strong Jovian magnetospheric plasma interaction with Ganymede and does not appear to be strongly influenced by the orientation of the background Jovian magnetic field. The observed auroral oval pattern is reasonably well matched by a magnetohydrodymanic (MHD) model optimized to fit the Galileo magnetic field measurements near Ganymede. The location of the auroral oval from these data provides a reasonable match to the location of the well‐defined visible boundary of the Ganymede polar cap except in the northern, leading hemisphere.

Model of the Jovian magnetic field topology constrained by the Io auroral emissions

[1] The determination of the internal magnetic field of Jupiter has been the object of many studies and publications. These models have been computed from the Pioneer, Voyager, and Ulysses measurements. Some models also use the position of the Io footprints as a constraint: the magnetic field lines mapping to the footprints must have their origins along Io's orbit. The use of this latter constraint to determine the internal magnetic field models greatly improved the modeling of the auroral emissions, in particular the radio ones, which strongly depends on the magnetic field geometry. This constraint is, however, not sufficient for allowing a completely accurate modeling. The fact that the footprint field line should map to a longitude close to Io's was not used, so that the azimuthal component of the magnetic field could not be precisely constrained. Moreover, a recent study showed the presence of a magnetic anomaly in the northern hemisphere, which has never been included in any spherical harmonic decomposition of the internal magnetic field. We compute a decomposition of the Jovian internal magnetic field into spherical harmonics, which allows for a more accurate mapping of the magnetic field lines crossing Io, Europa, and Ganymede orbits to the satellite footprints observed in UV. This model, named VIPAL, is mostly constrained by the Io footprint positions, including the longitudinal constraint, and normalized by the Voyager and Pioneer magnetic field measurements. We show that the surface magnetic fields predicted by our model are more consistent with the observed frequencies of the Jovian radio emissions than those predicted by previous models.

Power transmission and particle acceleration along the Io flux tube

Io's motion relative to the Jovian magnetic field generates a power of about 1012 W, which is thought to propagate as an Alfvén wave along the magnetic field line. This power is transmitted to the electrons, which will then precipitate and generate the observed auroral phenomena from UV to radio wavelengths. A more detailed look at this hypothesis shows some difficulties: Can the Alfvén waves escape the torus or are they trapped inside? Where and how are the particles accelerated? In which direction? Is there enough power transmitted to the particles to explain the strong brightness of the auroral emissions in UV, IR, visible, and radio? In other words, can we make a global, consistent model of the Io‐Jupiter interaction that matches all the observations? To answer these questions, we review the models and studies that have been proposed so far. We show that the Alfvén waves need to be filamented by a turbulent cascade process and accelerate the electrons at high latitude in order to explain the observations and to form a consistent scheme of the Io‐Jupiter interaction.

Galileo constraints on the secular variation of the Jovian magnetic field

The variations of the terrestrial magnetic field place important constraints on the behavior of the fluid in the Earth's magnetic dynamo. Jupiter is currently the only other planet for which magnetic measurements exist over a sufficiently long baseline to enable a study of the secular variation of the field to be undertaken. The average magnetic moment during the Galileo epoch was 4.334 ± 0.010 Gauss RJ3 or 1.584 ± 0.004 × 1020 Tm3. The tilt angle of the dipole was 9.71° ± 0.05°. We examine both the change in the magnetic field during the period in which Galileo operated in Jovian orbit and the change in the field between the initial observations with Pioneer 11 and the Galileo epoch. Neither approach definitively identifies secular change in the Jovian field but rather puts a limit on that rate of change. The only significant change is associated with the current imprecision of the International Astronomical Union‐defined System III period.

Monte Carlo modeling of Io’s [OI] 6300 Å and [SII] 6716 Å auroral emission in eclipse

We present a Monte Carlo (MC) model of [OI] 6300 Å and [SII] 6716 Å emission from Io entering eclipse. The simulation accounts for the 3-D distribution of SO 2, O, SO, S, and O 2 in Io’s atmosphere, several volcanic plumes, and the magnetic field around Io. Thermal electrons from the jovian plasma torus are input along the simulation domain boundaries and move along the magnetic field lines distorted by Io, occasionally participating in collisions with neutrals. We find that the atmospheric asymmetry resulting from varying degrees of atmospheric collapse across Io (due to eclipse ingress) and the presence of volcanoes contributes significantly to the unique morphology of the [OI] 6300 Å emission. The [OI] radiation lifetime of ∼134 s limits the emission to regions that have a sufficiently low neutral density so that intermolecular collisions are rare. We find that at low altitudes (typically <40 km) and in volcanic plumes (Pele, Prometheus, etc.) the number density is large enough (>4 × 10 9 cm −3) to collisionally quench nearly all (>95%) of the excited oxygen for reasonable quenching efficiencies. Upstream (relative to the plasma flow), Io’s perturbation of the jovian magnetic field mirrors electrons with high pitch angles, while downstream collisions can trap the electrons. This magnetic field perturbation is one of the main physical mechanisms that results in the upstream/downstream brightness asymmetry in [OI] emission seen in the observation by Trauger et al. (Trauger, J.T., Stapelfeldt, K.R., Ballester, G.E., Clarke, J.I., 1997. HST observations of [OI] emissions from Io in eclipse. AAS-DPS Abstract (1997DPS29.1802T)). There are two other main causes for the observed brightness asymmetry. First, the observation’s viewing geometry of the wake spot crosses the dayside atmosphere and therefore the wake’s observational field of view includes higher oxygen column density than the upstream side. Second, the phased entry into eclipse results in less atmospheric collapse and thus higher collisional quenching on the upstream side relative to the wake. We compute a location (both in altitude and latitude) for the intense wake emission feature that agrees reasonably well with this observation. Furthermore, the peak intensity of the simulated wake feature is less than that observed by a factor of ∼3, most likely because our model does not include direct dissociation–excitation of SO 2 and SO. We find that the latitudinal location of the emission feature depends not so much on the tilt of the magnetic field as on the relative north/south flux tube depletion that occurs due to Io’s changing magnetic latitude in the plasma torus. From 1-D simulations, we also find that the intensity of [SII] 6716 and 6731 Å emission is much weaker than that of [OI] even if the [SII] excitation cross section is 10 3 times larger than excitation to [OI]. This is because the density of S + is much less than that of O and because the Einstein- A coefficient of the [SII] emission is a factor of ∼10 smaller than that of [OI].

The Io UV footprint: Location, inter‐spot distances and tail vertical extent

The Io footprint (IFP) consists of one or several spots observed in both jovian hemispheres and is related to the electromagnetic interaction between Io and the magnetosphere. These spots are followed by an auroral curtain, called the tail, extending more than 90° longitude in the direction of planetary rotation. We use recent Hubble Space Telescope images of Jupiter to analyze the location of the footprint spots and tail as a function of Io's location in the jovian magnetic field. We present here a new IFP reference contour—the locus of all possible IFP positions—with an unprecedented accuracy, especially in previously poorly covered sectors. We also demonstrate that the lead angle ‐ the longitudinal shift between Io and the actual IFP position ‐ is not a reliable quantity for validation of the interaction models. Instead, the evolution of the inter‐spot distances appears to be a better diagnosis of the Io‐Jupiter interaction. Moreover, we present observations of the tail vertical profiles as seen above the limb. The emission peak altitude is ∼900 km and remains relatively constant with the distance from the main spot. The altitudinal extent of the vertical emission profiles is not compatible with precipitation of a mono‐energetic electron population. The best fit is obtained for a kappa distribution with a characteristic energy of ∼70 eV and a spectral index of 2.3. The broadness of the inferred electron energy spectrum gives insight into the physics of the electron acceleration mechanism at play above the IFP tail.

Modelling the Io‐related DAM emission by modifying the beaming angle

The Io–Jupiter interaction generates strong decametric radio emissions (DAM), which appear as arcs in the time–frequency plane. These emissions are beamed at an angle from the magnetic field lines, which may vary with frequency and longitude amongst other properties. Empirical models of this beaming angle describe the shape of the DAM arcs and offer insight into the emission mechanism for DAM. Several studies have investigated the variation in the emission beaming angle. The studies span a range of frequencies which depend on the observational means (spacecraft, ground‐based radio telescopes) used to obtain data. Subsequently, because of the varying assumptions made (e.g. relativistic vs. non‐relativistic electrons for the wave polarization), methods used (e.g. prescribing a beaming angle function vs. determining a beaming angle function from observational geometry) and frequency ranges observed, different results have been found in each study. In the present paper, we model the shape of the emission with an empirical beaming angle function and adjust the parameters to best fit the emission arcs. However, our model builds on previous models by taking into account the location of Io in the Jovian magnetic field. We also look at a broader frequency range than many of the intermediate studies. We find that a simple empirical beaming angle function describes the shape of the A, B, and D arcs and that the beaming angle function must decrease at high and low frequencies. We then propose a simple explanation for the beaming angle profile, deduced from cyclotron maser theory.

Dependence of the Io-related decametric radio emission of Jupiter on the central meridian longitude and Io's “active” longitudes

Context.The statistical analysis of the Io-related decametric radio emission of Jupiter shows that this emission depends precisely on the central meridian longitude. This dependence is the result of the existence of Io's “active” longitudes, i.e. particular regions of Io's orbit, which are fixed with respect to the Jovian magnetic field and at which Io-related emission occurs more often.

Morphology of the ultraviolet Io footprint emission and its control by Io's location

A total of 74 images of the ultraviolet footprint of the Io flux tube (IFT) on Jupiter's upper atmosphere made with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope have been analyzed to characterize their location, morphology, and brightness distribution. The observations cover a wide range of central meridian Jovian longitudes and Io orbital positions and include north and south footprint emissions. Comparing the location of the IFT with that expected from the VIP4 model of the Jovian magnetic field, we find that the lead angle is generally not significantly different from zero in the System III longitude sector 125°–195°. Instead, the lead angles reach about 8° in the 50° sector, coinciding with a region of possible magnetic anomaly. We observe that the brightness of the main footprint shows intrinsic intensity changes that appear to be controlled by the system III longitude of Io and its position above or below the center of the torus. The size of the primary spot magnetically maps into a region varying from 1 to over 10 Io diameters in Io's orbital plane. Multiple footprints are observed with varying brightness relative to the mean spot. The number of spots is found to increase as Io gets closer to the torus outer edge facing the spots. The separation between the first and second spots is typically 1°–3° of longitude and increases when Io is displaced from the torus center in the direction of the IFT signature. These features confirm that Alfvén waves play an important role and generate energization of precipitated electrons. However, the observed variation of the FUV spot structure with Io's position appears inconsistent with models where reflections of Alfvén wings occur between the torus boundary and Jupiter's ionosphere. Instead, the multiple spots apparently correspond to electron precipitation generated by Alfvén waves reflected inside the plasma torus.

Solar wind control of Jovian auroral emissions

The combination of Galileo/PWS and Wind/WAVES observations allows the study of the flux density variation of the Jovian hectometric emissions (HOM) observed from 31 August 1996 until 24 October 1996. It is found that the HOM emission presents periodic features; each one is called a “HOM event.” Such episodic emissions were concurrently observed by both experiments with similar spectral characteristics. The fluctuations of the Jovian hectometric emissions and the solar wind parameters are found to exhibit quasi‐similar variations when a time lag of about 153 days is taken into consideration. Also, “HOM enhancements,” like the “injection events” first reported by Mauk et al. (1997), are found to occur at some specific longitudes. The occurrence of these magnetospheric events increases at two “active longitudes,” i.e., 45° CML and 180° CML. The solar wind seems to be at the origin of both phenomena. Solar particles go through the polar regions where they interact with the Jovian magnetic field and give rise in the auroral and equatorial regions to an increase of Jovian hectometric emissions and/or injection events.

Structure of the Magnetic Field of the Jovian Magnetosphere

We exactly solved the problem of the interaction between the rotating magnetic field of Jupiter and the equatorial plasma disk formed by the gases flowing from the Jovian satellite Io. The disk is shown to expel the Jovian magnetic field in both directions, inward, toward Jupiter, compressing its dipole magnetic field, and outward. Jupiter spins up the disk up to velocities that correspond to nearly constant angular rotation, but with an angular frequency lower than the angular frequency of Jupiter itself. The radial velocity of the plasma in the disk approaches its azimuthal velocity. We determined the power of Jupiter’s rotational energy losses. Part of this energy is transferred to the disk, and the other part goes into heating the Jovian ionosphere. We show that the Pedersen surface conductivity of the Jovian ionosphere must have a lower limit to maintain the electric current that arises in the disk-rotating magnetic field system. This current in the Jovian magnetosphere flows only along the preferential magnetic surfaces that connect the inner and outer edges of the disk to the ionosphere.