•Water production rates from 3700 SOHO/SWAN H Ly-α images of 61 comets.•Determined pre- and post-perihelion power-law heliocentric distance dependencies.•Compared power-law exponents and active ...fractions to various dynamical quantities.•Evidence for evolution of cometary nuclei in both long-period and short period comets.
The Solar Wind Anisotropies (SWAN) instrument on the SOlar and Heliospheric Observatory (SOHO) satellite has observed 44 long period and new Oort cloud comets and 36 apparitions of 17 short period comets since its launch in December 1995. Water production rates have been determined from the over 3700 images producing a consistent set of activity variations over large parts of each comet's orbit. This has enabled the calculation of exponential power-law variations with heliocentric distance of these comets both before and after perihelion, as well as the absolute values of the water production rates. These various measures of overall water activity including pre- and post-perihelion exponents, absolute water production rates at 1 AU, active surface areas and their variations have been compared with a number of dynamical quantities for each comet including dynamical class, original semi-major axis, nucleus radius (when available), and compositional taxonomic class. Evidence for evolution of cometary nuclei is seen in both long-period and short-period comets.
Abstract
Interstellar neutral hydrogen flows into the heliosphere as a mixture of the primary and secondary populations from two somewhat different directions due to splitting occurring in the ...magnetized outer heliosheath. The direction of the inflow of interstellar neutral H observed in the inner heliosphere, confronted with that of the unperturbed flow of interstellar neutral helium, is important for understanding the geometry of the distortion of the heliosphere from axial symmetry. It is also needed for facilitating remote-sensing studies of the solar wind structure based on observations of the helioglow, such as those presently performed by SOHO/SWAN, and in the near future by IMAP/GLOWS. In the past, the only means to measure the direction of the flow of interstellar hydrogen were spectroscopic observations of the helioglow. Here, we propose a new method to determine this parameter based on a long series of photometric observations of the helioglow. The method is based on purely geometric considerations and does not depend on any model and absolute calibration of the measurements. We apply this method to sky maps of the helioglow available from the SOHO/SWAN experiment and derive the mean longitude of the flow of interstellar hydrogen. We obtain 253.°1 ± 2.°8, which is in perfect agreement with the previously obtained results based on spectroscopic observations.
We analyze SOHO (SOlar Heliospheric Observatory)/SWAN (Solar Wind ANisotropy) hydrogen Lyman‐α data collected between 1996 and 2018 to derive the solar wind latitudinal distribution over time. ...Full‐sky interplanetary Lyman‐α maps are inverted to derive the total hydrogen ionization rate latitude profiles, normalized to proton charge‐exchange and photoionization. Using Interplanetary Scintillation velocities to calculate the velocity‐dependent charge‐exchange cross‐sections, we produce the solar wind flux latitudinal profiles. Finally, we compute solar wind velocity latitude profiles, based on the dynamic pressure and energy flux conservation (calculated from OMNI data) over latitude. SWAN reproduces the Interplanetary Scintillation velocity profiles up to at least ±60°, and also agrees with Ulysses in situ measurements for solar minimum periods in 1996–1997 and 2007. During solar maximum, discrepancies are more frequent because in situ data reflect local solar wind conditions, while SWAN data reflect global conditions in the heliosphere.
Key Points
The SOHO/SWAN instrument is one of the few to derive the global solar wind distribution outside the ecliptic
The data analysis produces latitude distributions for the solar wind particle flux and velocity as a function of time
Comparison with IPS and Ulysses velocity data confirms the conservation of dynamic pressure and energy flux in the solar wind
The Earth's hydrogen exosphere Lyman‐α radiation was mapped with the Solar Wind Anisotropies/Solar and Heliospheric Observatory (SWAN/SOHO) instrument in January 1996, 1997, and 1998 (low solar ...activity). The use of a hydrogen absorption cell allowed to disentangle the interplanetary emission from the geocoronal one and to assign the absorbed signal almost entirely to the geocorona. The geocorona was found to extend at least up to 100 Earth radii (RE) with an intensity of 5 Rayleigh, an unprecedented distance well exceeding the recent results of Lyman Alpha Imaging Camera (LAICA) imager (∼50 RE), and encompassing the orbit of the Moon (∼60 RE). We developed a numerical kinetic model of the hydrogen atoms distribution in the exosphere, which includes the solar Lyman‐α radiation pressure and the ionization. The radiation pressure compresses the H exosphere on the dayside, producing a bulge of H density between 3 and 20 RE, which fits observed intensities very well. The SWAN Lyman‐α distribution of intensity was compared both to LAICA (2015) and to Orbiting Geophysical Observatory number 5 (1968) measurements. Integrated H densities of SWAN at a tangent distance of 7 RE are larger than LAICA/Orbiting Geophysical Observatory number 5 by factors 1.1–2.5, while we should expect a stronger effect of the radiation pressure at solar max. We discuss the possible role of H atoms in satellite orbits to explain this apparent contradiction. An onion‐peeling technique is used to retrieve hydrogen number density in the exosphere for the three SWAN observations. They show an excess of density versus models at large distances, which is likely due to nonthermal atoms (not in the model).
Key Points
We find that the geocorona extends to almost twice the distance of the Moon
the H exosphere is compressed by solar radiation pressure, forming a bulge on the dayside
this bulge is enhanced at low solar activity, possibly in relation with a population of Hatoms in satellite orbits
•SOHO SWAN obtained 885 images of H Lyman-α 9 dynamically new and long period comets.•Water production rates were calculated for all comets.•Pre- and post-perihelion activity variations were ...characterized for each comet.•Comet C/2014 Q1 (PanSTARRS) showed evidence of massive shedding and sublimation of nucleus material around perihelion.•It is estimated that C/2014 Q1 (PanSTARRS) lost ∼64% of its original mass.
Nine recently discovered long-period comets were observed by the Solar Wind Anisotropies (SWAN) Lyman-alpha all-sky camera on board the Solar and Heliosphere Observatory (SOHO) satellite during the period of 2013 to 2016. These were C/2012 K1 (PanSTARRS), C/2013 US10 (Catalina), C/2013 V5 (Oukaimeden), C/2013 R1 (Lovejoy), C/2014 E2 (Jacques), C/2014 Q2 (Lovejoy), C/2015 G2 (MASTER), C/2014 Q1 (PanSTARRS) and C/2013 XI (PanSTARRS). Of these 9 comets 6 were long-period comets and 3 were possibly dynamically new. Water production rates were calculated from each of the 885 images using our standard time-resolved model that accounts for the whole water photodissociation chain, exothermic velocities and collisional escape of H atoms. For most of these comets there were enough observations over a broad enough range of heliocentric distances to calculate power-law fits to the variation of production rate with heliocentric distances for pre- and post-perihelion portions of the orbits. Comet C/2014 Q1 (PanSTARRS), with a perihelion distance of only ∼0.3 AU, showed the most unusual variation of water production rate with heliocentric distance and the resulting active area variation, indicating that when the comet was within 0.7 AU its activity was dominated by the continuous release of icy grains and chunks, greatly increasing the active sublimation area by more than a factor of 10 beyond what it had at larger heliocentric distances. A possible interpretation suggests that a large fraction of the comet's mass was lost during the apparition.
Observations of the Lyman‐α emissions from Interplanetary Hydrogen (IPH) atoms are made from Mars' orbit using a high spectral resolution instrument in echelle configuration. The measurements can ...uniquely be used to resolve IPH from planetary H emissions and to subsequently determine the brightness, velocity, and thermal broadening of the IPH flow along the instrument line of sight. Planned observations conducted during special IPH campaigns as well as serendipitous observations made of the planetary limb and a comet sighting, both upwind and downwind of the bulk IPH flow direction, are analyzed to determine these properties and to examine the variability of IPH brightness with solar activity through the declining phase of Solar Cycle 24. The results show that the IPH brightness trends with solar irradiance, the flow is fainter downwind than upwind, the IPH brightness is variable and non‐negligible compared with planetary emissions, and that deriving thermal properties of IPH requires higher spectral resolution than is presently available. A heliospheric interface model was used to simulate and further interpret the derived IPH properties. These results can improve the theoretical understanding of solar system dynamics, between the solar wind and the local interstellar medium, by providing empirical constraints to simulations of the global heliosphere from the inner boundary region near 1.6 AU and can guide the development of future interplanetary missions.
Key Points
Interplanetary Hydrogen (IPH) Lyman‐a brightness trends with solar irradiance with fainter flow downwind than upwind
IPH Lyman‐a brightness is variable and non‐negligible when viewed from Mars‐orbiting spacecraft
MAVEN IPH observations can be used to constrain the inner boundary conditions of heliospheric models
In this work, we present for the first time the Lyman α intensities measured by Voyager 1/UVS in 2003–2014 (at 90–130 AU from the Sun). During this period Voyager 1 measured the Lyman α emission in ...the outer heliosphere at an almost fixed direction close to the upwind (i.e.“ toward the interstellar flow). The data show an unexpected behavior in 2003–2009: the ratio of observed intensity to the solar Lyman α flux is almost constant. Numerical modeling of these data is performed in the frame of a state‐of‐the‐art self‐consistent kinetic‐MHD model of the heliospheric interface. The model results, for various interstellar parameters, predict a monotonic decrease of intensity not seen in the data. We propose two possible scenarios that explain the data qualitatively. The first is the formation of a dense layer of hydrogen atoms near the heliopause. Such a layer would provide an additional backscattered Doppler‐shifted Lyman α emission, which is not absorbed inside the heliosphere and may be observed by Voyager. About 35 R of intensity from the layer is needed. The second scenario is an external nonheliospheric Lyman α component, which could be galactic or extragalactic. Our parametric study shows that ∼25 R of additional emission leads to a good qualitative agreement between the Voyager 1 data and the model results.
Key Points
For the first time we present the Lyman‐alpha intensities measured by Voyager 1/UVS in 2003–2014 (at 90–130 AU from the Sun)
The data show an unexpected flat behavior at 90–115 AU
An additional source of emission is necessary to fit the data; we suggest two possible qualitative scenarios that can explain the data
Abstract
C/2020 F3 (NEOWISE) was discovered in images from the Near Earth Object program of the Wide-Field Infrared Survey Explorer (NEOWISE) taken on 2020 March 27 and has become the Great Comet of ...2020. The Solar Wind ANisotropies (SWAN) camera on the Solar and Heliospheric Observatory (SOHO) spacecraft, located in a halo orbit around the Earth–Sun L1 Lagrange point, makes daily full-sky images of hydrogen Ly
α
. Water production rates were determined from the SWAN hydrogen Ly
α
brightness and spatial distribution of the comet measured over a 4 month period of time on either side of the comet’s perihelion on 2020 July 3. The water production rate in s
−1
was moderately asymmetric around perihelion and varied with the heliocentric distance,
r
, in au as (6.9 ± 0.5) × 10
28
r
−2.5±0.2
and (10.1 ± 0.5) × 10
28
r
−3.5±0.1
before and after perihelion, respectively. This is consistent with the comet having been through the planetary region of the solar system on one or more previous apparitions. Water production rates as large as 5.27 × 10
30
s
−1
were determined shortly after perihelion, once the comet was outside the solar avoidance area of SWAN, when the comet was 0.324 au from the Sun.
Using an absorption cell, we measured the Doppler shifts of the interstellar hydrogen resonance glow to show the direction of the neutral hydrogen flow as it enters the inner heliosphere. The neutral ...hydrogen flow is found to be deflected relative to the helium flow by about 4°. The most likely explanation of this deflection is a distortion of the heliosphere under the action of an ambient interstellar magnetic field. In this case, the helium flow vector and the hydrogen flow vector constrain the direction of the magnetic field and act as an interstellar magnetic compass.
The Solar Wind ANisotropies (SWAN) all-sky hydrogen Ly camera on the SOlar and Heliospheric Observer satellite observed the hydrogen coma of comet C/2017 S3 (PanSTARRS) for the last month of its ...activity from 2018 July 4 to August 4 and what appears to have been its final disintegration just 11 days before its perihelion on August 15. The hydrogen coma indicated water production had a small outburst on July 8 at a heliocentric distance of 1.1 au and then a much larger one on July 20 at 0.8 au. Over the following two weeks the water production dropped by more than a factor of 10 after which it was no longer detectable. The behavior is reminiscent of comet C/1999 S4 (LINEAR) in 2000, which had a few small outbursts on its inbound orbit and a major outburst at a heliocentric distance of about 0.8 au, which was close to its perihelion, followed by its complete disintegration that was documented by several sets of observations including SWAN. C/2017 S3 (PanSTARRS), however, had a much larger water production rate than C/1999 S4 (LINEAR). Here we estimate the size of the nucleus of C/2017 S3 just before its final outburst and apparent disintegration were estimated using the total amount of water produced during its last weeks for a range of values of the refractory/ice ratio in the nucleus. We also determine the size distribution of the disintegrating particles as the comet faded.
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