Breakup of moving clusters in the galactic disk

Numerical simulations are used to analyze the breakup of moving groups in the Galactic disk through the internal velocity dispersion of the group components and the tidal effect of the external regular gravitational Galactic field and giant molecular clouds. The initial locations of the group centroids correspond to well-known moving streams: the Hyades, the Pleiades, the Ursa Major cluster, and the group HR 1614. The mean group breakup times have been found as a function of the adopted limiting group size. The interactions of stream stars with giant molecular clouds reduce significantly the group lifetime.     

Observations of Polar Plumes with the Sumer Instrument on Soho

We present new observations of O vi 1032 Å line profiles in polar plumes, and inter-plume regions, on the disk and above the limb in the north coronal hole obtained with the SUMER (Solar Ultraviolet Measurements of Emitted Radiation) instrument on the SOHO (Solar and Heliospheric Observatory) spacecraft. On 22 May 1996, a 5 x 5 arc min spectroheliogram was scanned above the north polar coronal hole with the entrance slit extending from 1.03 to 1.33 solar radii with 1.5 arc sec spatial resolution and ≈ 0.044 Å per pixel spectral resolution in the wavelength range 1020–1040 Å. Detailed plume structure in O vi 1032 Å can be seen extending beyond 1.3 solar radii, with intensities in the plume regions 10–50% brighter, but line widths 10–15% narrower, than the inter-plume regions. Possible explanations for this observed anti-correlation between line width and intensity in the plume and inter-plume regions are discussed. We conclude that the source of the high-speed solar wind may not be polar plumes, but the inter-plume lanes associated with open magnetic field regions of the chromospheric network.     

Rhessi and Trace Observations of the 21 April 2002 x1.5 Flare

Solar Physics - Observations of the X1.5 flare on 21 April 2002 are reviewed using the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and the Transition Region and Coronal Explorer...     

Solar oscillations and the equation of state

Summary Accurate measurements of observed frequencies of solar oscillations are providing a wealth of data on the properties of the solar interior. The frequencies depend on solar structure, and on the properties of the plasma in the Sun. Here we consider in particular the dependence on the thermodynamic state. From an analysis of the equations of stellar structure, and the relevant aspects of the properties of the oscillations, we argue that in the convection zone one can isolate information about the equation of state which is relatively unaffected by other uncertainties in the physics of the solar interior. We review the different treatments that have been used to describe the thermodynamics of stellar plasmas. Through application of several of these to the computation of models of the solar envelope we demonstrate that the sensitivity of the observed frequencies is in fact sufficient to distinguish even quite subtle features of the physics of solar matter. This opens up the possibility of using the Sun as a laboratory for statistical mechanics, under conditions that are out of reach in a terrestrial laboratory.     

Split Comet C/2001 A2 (LINEAR)

Comet C/2001 A2 experienced several splitting events duringits 2001 perihelion passage. The first break-upevent was observed in March 2001 (IAUC 7616).In this paper we report the first results of ourextensive imaging and spectroscopic monitoring campaignwith ESO telescopes over several weeks before andafter the perihelion passage on May 25 2001.     

Comparative Study of the Optical Spectra of Comets C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp)

Earth, Moon, and Planets -     

The Source Regions of Impulsive Solar Electron Events

Low-energy (2–19 keV) impulsive electron events observed in interplanetary space have been traced back to the Sun, using their interplanetary type III radiation and metric/decimetric radio-spectrograms. For the first time we are able to study the highest frequencies and thus the radio signatures closest to the source region. All the selected impulsive solar electron events have been found to be associated with an interplanetary type III burst. This allows to time the particle events at the 2 MHz plasma level and identify the associated coronal radio emissions. Except for 5 out of 27 cases, the electron events were found to be associated with a coronal type III burst in the metric wavelength range. The start frequency yields a lower limit to the density in the acceleration region. We also search for narrow-band spikes at the start of the type III bursts. In about half of the observed cases we find metric spikes or enhancements of type I bursts associated with the start of the electron event. If interpreted as the plasma emission of the acceleration process, the observed average frequency of spikes suggests a source density of the order of 3×10⁸ cm^–3 consistent with the energy cut-off observed.     

Modeling Images and Spectra of a Solar Flare Observed by RHESSI on 20 February 2002

We have analyzed a C7.5 limb flare observed by RHESSI on 20 February 2002. The RHESSI images appear to show two footpoints and a loop-top source. Our goal was to determine if the data are consistent with a simple steady-state model in which high-energy electrons are continuously injected at the top of a semicircular flare loop. A comparison of the RHESSI images with simulated images from the model has made it possible for us to identify spurious sources and fluxes in the RHESSI images. We find that the RHESSI results are in many aspects consistent with the model if a thermal source is included between the loop footpoints, but there is a problem with the spectral index of the loop-top source. The thermal source between the footpoints is likely to be a low-lying loop interacting with the northern footpoint of a higher loop containing the loop-top source.     

Biogeomorphology,quo vadis? On processes,time, and space in biogeomorphology

Biogeomorphology has been expanding as a discipline, due to increased recognition of the role that biology can play in geomorphic processes, as well as due to our increasing capacity to measure and quantify feedback between biological and geomorphological systems. Here, we provide an overview of the growth and status of biogeomorphology. This overview also provides the context for introducing this special issue on biogeomorphology, and specifically examines the thematic domains of biogeomorphological research, methods used, open questions and conundrums, problems encountered, future research directions, and practical applications in management and policy (e.g. nature-based solutions). We find that whilst biogeomorphological studies have a long history, there remain many new and surprising biogeomorphic processes and feedbacks that are only now being identified and quantified. Based on the current state of knowledge, we suggest that linking ecological and geomorphic processes across different spatio-temporal scales emerges as the main research challenge in biogeomorphology, as well as the translation of biogeomorphic knowledge into management approaches to environmental systems. We recommend that future biogeomorphic studies should help to contextualize environmental feedbacks by including the spatio-temporal scales relevant to the organism(s) under investigation, using knowledge of their ecology and size (or metabolic rate). Furthermore, in order to sufficiently understand the ‘engineering’ capacity of organisms, we recommend studying at least the time period bounded by two disturbance events, and recommend to also investigate the geomorphic work done during disturbance events, in order to put estimates of engineering capacity of biota into a wider perspective. Finally, the future seems bright, as increasingly inter-disciplinary and longer-term monitoring are coming to fruition, and we can expect important advances in process understanding across scales and better-informed modelling efforts. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd