The Structure and Motions of the 3C~120 Radio Jet on Scales of 0.6 to 300 Parsecs

Results are presented from long term VLBI monitoring of the parsec-scale radio jet in 3C120, primarily at 5 and 1.7 GHz. Numerous superluminal features are followed, some over large fractional distances from the core. The speeds of the features are not all the same, but vary by less than a factor of 2. The superluminal features extend to core distances of over 0.2 arcseconds and the region of slowing has not been found. The 1.7 GHz images show evidence for stationary features in addition to the superluminal features. The structure of the jet in the vicinity of the most likely stationary feature is suggestive of a helical pattern seen in projection. The deprojected wavelength of the pattern is very long relative to the jet radius, unlike the situation in sources such as M87. If the 3C120 jet does contain a slowly-moving, helical structure, then theory suggests that the jet resides in a relatively cool medium, not in a relativistically hot cocoon or lobe.Comment: 35 pages with 8 postscript figures. Accepted for publication in the Astrophysical Journa

Is the Jovian auroral H

Context. Measurement of linear polarisation in Earth’s thermospheric oxygen red line can be a useful observable quantity for characterising conditions in the upper atmosphere; therefore, polarimetry measurements are extended to other planets. Since FUV emissions are not observable from the ground, the best candidates for Jupiter auroral emissions are \hbox{$\textrm{H}_{3}^{+}$} infrared lines near 4 μm. This ion is created after a chemical process in the Jovian upper atmosphere. Thus the anisotropy responsible of the polarisation cannot be the particle impact as in the Earth case. Aims. The goal of this study is to detect polarisation of \hbox{$\textrm{H}_{3}^{+}$} emissions from Jupiter’s aurora. Methods. Measurements of the \hbox{$\textrm{H}_{3}^{+}$} emissions from Jupiter’s southern auroral oval were performed at the UK Infrared Telescope using the UIST-IRPOL spectro-polarimeter, with the instrument slit positioned perpendicular to Jupiter’s rotation axis. Data were processed by dividing the slit into 24 bins. Stokes parameters (u, q and v), polarisation degree and direction were extracted for each bin and debiased. Results. More than 5 bins show polarisation with a confidence level above 3σ. Polarisation degrees up to 7% are detected. Assuming the auroral intensity is constant during the 8 waveplate positions exposure time, i.e. around 10 min, strong circular polarisation is present, with an absolute value of the Stokes v parameter up to 0.35. Conclusions. This study shows that polarisation is detectable in the Jovian infrared auroras, but new measurements are needed to be able to use it to characterise the ionospheric environment. At present, it is not possible to propose a mechanism to explain this polarisation owing to the lack of theoretical work and laboratory experiments concerning the polarisation of \hbox{$\textrm{H}_{3}^{+}$}

Nightside reconnection at Jupiter: Auroral and magnetic field observations from 26

In this study we present ultraviolet and infrared auroral data from 26 July 1998, and we show the presence of transient auroral polar spots observed throughout the postdusk to predawn local time sector. The polar dawn spots, which are transient polar features observed in the dawn sector poleward of the main emission, were previously associated with the inward moving flow resulting from tail reconnection. In the present study we suggest that nightside spots, which are polar features observed close to the midnight sector, are related to inward moving flow, like the polar dawn spots. We base our conclusions on the near-simultaneous set of Hubble Space Telescope (HST) and Galileo observations of 26 July 1998, during which HST observed a nightside spot magnetically mapped close to the location of an inward moving flow detected by Galileo on the same day. We derive the emitted power from magnetic field measurements along the observed plasma flow bubble, and we show that it matches the emitted power inferred from HST. Additionally, this study reports for the first time a bright polar spot in the infrared, which could be a possible signature of tail reconnection. The spot appears within an interval of 30 min from the ultraviolet, poleward of the main emission on the ionosphere and in the postdusk sector planetward of the tail reconnection x line on the equatorial plane. Finally, the present work demonstrates that ionospheric signatures of flow bursts released during tail reconnection are instantaneously detected over a wide local time sector

Evolution of Saturn's Bright Polar Aurora

Observations of Saturn's infrared aurorae have shown that in addition to the main auroral oval, which is believed to be associated with the solar wind, there are significant polar emissions. Ground-based infrared observations of Saturn have been able to show that there is a general level of raised emission across the entire polar region, in a similar way to that seen at Jupiter. However, with direct observations of the aurora made from orbit around Saturn by the Cassini-VIMS instrument, this aurora was shown to be more than a relative generalised brightening in the infrared. Instead, a unique auroral feature was observed to occur, appearing as a large region of bright polar emission, positioned poleward of 82 degrees latitude. This Bright Polar Aurora emission is significantly different from the recently observed subrotating Q-branch auroral emission seen in both the ultraviolet and infrared, as it is separated from the main auroral oval by a region of low emission. This effectively produces a cap of bright aurora inside the main auroral oval, surrounded by a dark ring that separates the two aurorae. Here, we take a more detailed look at this cap of emission and examine the way the auroral feature develops with time. Bright Polar Aurora emission has been observed in two separate VIMS images. A more detailed analysis of the polar emission shows that each of these images in fact differs in structure; the first has auroral emission across the whole polar cap >82 degrees, but within the second the emission is concentrated on the dusk side. While the dramatic in-filling of the polar cap is not seen within any UV observations, the Hubble Space Telescope has observed transitory duskward auroral features within the polar cap, in a similar location to the duskward feature seen in the infrared. Using ground-based infrared observations, which allow a Bright Polar Aurora event to be broken into shorter timescale steps, it is possible analyse the progression of the infrared auroral emission with time, connecting the morphology seen within the two VIMS images with those in the ultraviolet

Nightside reconnection at Jupiter: Auroral and magnetic field observations from 26 July 1998

In this study we present ultraviolet and infrared auroral data from 26 July 1998, and we show the presence of transient auroral polar spots observed throughout the postdusk to predawn local time sector. The polar dawn spots, which are transient polar features observed in the dawn sector poleward of the main emission, were previously associated with the inward moving flow resulting from tail reconnection. In the present study we suggest that nightside spots, which are polar features observed close to the midnight sector, are related to inward moving flow, like the polar dawn spots. We base our conclusions on the near-simultaneous set of Hubble Space Telescope (HST) and Galileo observations of 26 July 1998, during which HST observed a nightside spot magnetically mapped close to the location of an inward moving flow detected by Galileo on the same day. We derive the emitted power from magnetic field measurements along the observed plasma flow bubble, and we show that it matches the emitted power inferred from HST. Additionally, this study reports for the first time a bright polar spot in the infrared, which could be a possible signature of tail reconnection. The spot appears within an interval of 30 min from the ultraviolet, poleward of the main emission on the ionosphere and in the postdusk sector planetward of the tail reconnection x line on the equatorial plane. Finally, the present work demonstrates that ionospheric signatures of flow bursts released during tail reconnection are instantaneously detected over a wide local time sector

Euclid mission status after mission critical design

International audienceEuclid, an ESA mission designed to characterise dark energy and dark matter, passed its Mission Critical Design Review in November 2018. It was demonstrated that the project is ready to start integration and test of the main systems, and that it has the ability to fulfil its top-level mission requirements. In addition, based on the performances at M-CDR, the scientific community has verified that the science requirements can be achieved for the Weak Lensing and Galaxy Clustering dark energy probes, namely a dark energy Figure of Merit of 400 and a 2% accuracy in the growth factor exponent gamma. We present the status of the main elements of the Euclid mission in the light of the demanding high optical performance which is the essential design driver is the to meet the scientific requirements. We include the space segment comprising of a service module and payload module hosting the telescope and its two scientific instruments, and the ground segment, which encompasses the operational and science ground segment. The elements for the scientific success of the mission for a timely release of the data are shortly presented: the processing and calibration of the data, and the design of the sky survey. Euclid is presently on schedule for a launch in September 2022