Sun-Earth Line Research Articles

Space Weather Diamond: a four spacecraft monitoring system

We report here preliminary results of a mission analysis for a space weather monitoring system that provides continuous transmission of solar wind conditions 0.10 A.U. upstream from Earth. The system is based on four platforms that are phased into eccentric heliocentric orbits but, from the perspective of a fixed Sun–Earth line, the spacecraft appear to orbit Earth. This system offers a 10× improvement in reporting solar wind plasma and magnetic field characteristics beyond similar platforms located at the Lagrangian L-1 point. We describe launch and energy considerations, along with a preliminary analysis of communication requirements. The Space Weather Diamond offers significant potential for scientific insight into problems requiring coordinated observations from multiple vantage points by providing the ability to separate spatial from temporal variations. We discuss examples for payloads including both in situ and remote sensing instrumentation.

A continuous view of the dawn‐dusk polar cap

Continuous monitoring of airglow and auroral emissions over Eureka (89° magnetic latitude) through each winter has shown that the dusk and dawn boundaries of the polar cap can be routinely seen when the IMF Bz is northward. When this condition prevails the auroral oval shrinks poleward, and the polar boundaries are clearly and continuously seen in photometric dusk‐dawn meridian scans of OI 630 nm emission brightness. This fact is illustrated with two 24‐hr days of records on December 19, 1996 and January 19, 1998. These examples show, and others similarly, that polar arcs emerge from the dusk or dawn flanks of the auroral oval (depending on the sign of By) and thus occur on closed field lines. In a few cases polar arcs emerge from the midnight or noon sector of the oval and extend along the sun‐earth line to cross the Eureka meridian, bifurcating the polar cap. This new technique for monitoring the polar cap dusk‐dawn extent and all auroral activity within promises to be valuable for diagnostic studies of both polar auroral activity and magnetospheric topology during solar wind changes and major sun‐earth events.

Here comes Solar Probe!

Despite recent advances, fundamental questions remain about the nature of the solar corona and the solar wind: 1) What heats the corona and accelerates the solar wind? 2) Where do the different types of solar wind originate? 3) Where and how are energetic particles produced and transported near the Sun? 4) What role do plasma turbulence and waves play in the corona and solar wind production? 5) What is the nature of the magnetic field and photospheric structures near the solar poles? Flying a trajectory perpendicular to the Earth-Sun line during its perihelion passage, Solar Probe will use in-situ and imaging instruments to provide the first three dimensional viewing of the corona, direct observations of solar polar regions, and local sampling of the solar environment. These primary observations are complemented by context-setting measurements and Earth-based observations. Solar Probe is currently scheduled for launch in February 2007 as the third in the new Outer Planets/Solar Probe mission line of NASA and will arrive at the Sun in 2010 under solar maximum conditions with a second closest approach near solar minimum in 2015.

Apparent Latitudinal Modulation of the Solar Neutrino Flux

We examine the solar neutrino Nux, as measured by the Homestake neutrino detector, to search for evidence of a dependence upon the solar latitude of the Earth-Sun line that varies from south in 7i.25 mid-March to north in mid-September. Although the Nux does not obviously show any dependence 7i.25 on latitude, we do -nd evidence for a dependence of the variance of the Nux upon latitude. When data from 108 runs of the Homestake experiment are divided into four quartiles, sorted according to latitude, we -nd that the northernmost quartile exhibits a larger variance than the other three. By applying the shuffle test, we estimate the probability that this could have occurred by chance to be in the range 1%E2%. For more detailed information, we examine a reconstructed Nux II formed from our recent maximum likelihood spectrum analysis. This procedure indicates that the variance is largest at about north. We 6i.5 also -nd that the spectrum of the variance of the reconstructed Nux has a notable peak at 1 cycle y~1 tending to con-rm a latitude dependence of the variance. We also examine the 12.88 cycle yr periodicity described in our recent paper and -nd that the amplitude of the periodicity is greater for the north- ernmost quartile than for the other quartiles. We suggest that these e†ects may be attributed to resonant spin-Navor precession of left-handEhelicity electron neutrinos in the magnetic -eld of the solar radiative zone. Subject headings: Sun: interior E Sun: magnetic -elds E Sun: particle emission

Stereoscopic Observations of Solar Hard X‐Ray Flares Made byUlyssesandYohkoh

The Solar X-Ray/Cosmic Gamma-Ray Burst Experiment aboard the interplanetary spacecraft Ulysses has provided extensive observations of solar hard X-ray flares from a variety of angles with respect to the Sun-Earth line. During the period 1991 October-1993 June, Ulysses observed 13 flares that were also observed by the X-ray instruments aboard the Japanese satellite Yohkoh located near Earth. At least 12 flares were in full view of both the spacecraft. Eight flares, for which hard X-ray spectra were available, are examined to determine the directivity of the 20-125 keV hard X-ray sources in solar flares. They include one flare for which the view angles of Ulysses and Yohkoh were 80° and 25°, respectively. No evidence of systematic directivity was found within the uncertainty (a factor of ~2) of these measurements. These and other observations of directivity at higher energies are consistent with a nearly isotropic distribution of energetic electrons in most solar flares.

Gross deformation of the dayside magnetopause

On July 24, 1996, the entire magnetosheath moved past the Interball‐1 spacecraft twice within 7 min. Such rapid transits through the magnetosheath suggest the passage of an antisunward‐moving boundary wave with an amplitude in excess of 5 RE. A wave of this amplitude requires an order of magnitude decrease in the pressure confining the magnetosphere. Such decreases have been reported only in conjunction with hot flow anomalies, features accompanying interplanetary magnetic field discontinuities, bounded by narrow shocks, and identified on the basis of heated deflected flows transverse to the Earth‐Sun line. We identify all these features just outside the Interball‐1 magnetopause crossings on July 24, 1996.

Predicting the strength of the ring current

The Dessler-Parker-Sckopke relationship states that the fractional depression in the magnetic field averaged over the surface of a non-conducting Earth is 2 3 of the ratio of the energy of the charged particles trapped in the Earth's magnetic field to the energy of that magnetic field above the surface of the Earth. When the interior conductivity of the Earth is included in the calculation, a moderate storm with a 100 nT depression corresponds to a ring current whose total energy is 2.8 × 10 15J. Nevertheless deducing the ring current strength from ground magnetograms is not quite as simple as it would seem from the DPS relationship. The ring current is often not symmetric and the observing stations are not uniformly situated, nor are there enough of them. Accurate baselines are required and the strength of the quiet day ring current is unknown. Despite these uncertainties, disturbances in the Dst index can be predicted quite well when accurate measurements of the solar wind and interplanetary magnetic field that strike the magnetosphere are available, by use of the formula of Burton et al. (1975). In this model the field on the surface of the Earth has two components: the magnetopause current flowing on the surface of the magnetopause and the ring current in the equatorial regions. The magnetopause current is taken to be proportional to the square root of the solar wind dynamic pressure. The ring current is energized by the solar wind east-west electric field and de-energized through an exponential decay of the energy into the upper atmosphere. This simple formula works quite well despite the fact that the solar wind input to the magnetosphere must be controlled by additional factors such as the dynamic pressure of the solar wind and the electrical conductivity of the ionosphere. Improvements are needed in order to increase the advance warning time for solar wind conditions reaching the Earth. To achieve such an improvement requires multipoint measurements near the Earth-sun line. These measurements should be both continuous and significantly closer to the Sun. Such measurements are technically feasible today and affordable using solar-sailing “sentinels” carrying a small payload consisting of a solar wind analyzer and magnetometer, and possibly energetic particles.

Large Scale Dynamics of the Magnetospheric Tail Induced by Substorms: A Multisatellite Study

The large scale dynamics of the tail during substorms are examine during multi-satellite measurements made between 18 and 22:00 UT on August 22, 1983. We use data from ISEE1/2, 1977-007, IMP-8, and ISEE-3 located in the solar wind or tail at distances along the Sun-Earth line ranging from +11 to -205RE. During the period studied, which corresponded to a southward directed IMF, successive plasma injections were detected at 6.6RE first during a general increase of the magnetic field in the lobe. The magnetic field increase is attributed to an enhanced reconnection rate, as evidenced by the close relationship between the tail lobe magnetic energy density and the available Poynting energy in the solar wind, for near constant solar wind pressure. A Traveling Compression Region (flux rope) was detected in close association with the first energetic particle injection at geostationary orbit. After the first injections, the series of injections continued and the last ones are shown to be linked with a tailward propagating cross-tail current disruption detected onboard IMP-8 at ∼35RE. This event is seen in the distant tail at 205RE onboard ISEE-3 as an increase of the tail radius; the far magnetosphere engulfed the ISEE-3 satellite which was located in the solar wind prior to the event. The characteristic time delay between cross-tail current disruption onset occurring in the near-Earth plasma sheet and the enhancement of the tail diameter at 205RE is on the order of 40 minutes. Several hours before this event, a similar case was detected in which partial cross-tail current disruption in the mid-tail was also related to a far tail expansion. In both cases, the observed signatures are in agreement with the concept of the ejection of plasmoids from the magnetospheric tail with velocities comparable to that of the solar wind. These results strongly suggest that traveling cross-tail current disruption and plasmoid ejection from the far tail are the signature of the same propagating plasma process.

Stereoscopic viewing of solar coronal and interplanetary activity

The object of this presentation is to propose a concept for a space mission that would facilitate stereoscopic observations of the sun and interplanetary space, and to outline its scientific objectives. It is envisaged to place a spacecraft with a few lightweight telescopes, covering the wavelength range from the visible to the extreme UV, into orbit around the triangular libration point L5 of the sun-earth system. This point is also known as one of the Lagrangian points, which is in earth orbit around the sun, and lagging the sun-earth line by 60 degrees. A satellite (or perhaps more than one) with similar instrumentation is assumed to operate simultaneously in near earth orbit. Together, these observations would allow three dimensional viewing of the sun and interplanetary space. One of the main subjects of investigation would be to study the morphology and dynamics of coronal plasma structures and their extension into interplanetary space. Coronal structures would be imaged in EUV and soft X rays, while interplanetary events such as Coronal Mass Ejections (CMEs) and shock waves would be seen as density enhancements in white light. CMEs could be tracked from their start in the solar corona until their arrival and impact on the earth's magnetosphere. Thus the observations could also be used to predict major geomagnetic storms.

Nature and origin of wave modes in the dayside earth magnetosheath

The day side magnetosheath displays turbulence at frequencies lower than the local proton frequency. The nature of the waves modes is studied from ISEE-1-2 electron density and magnetic field data. We compare the polarization and the compressibility of the observed fluctuations to the polarization and the compressibility of the linear wave modes in a Maxwellian kinetic plasma model with an anisotropic proton temperature. Downstream of the undershoot of a supercritical quasi-perpendicular shock we successively observe Alfvén Ion Cyclotron (AIC) modes, mirror modes and then AIC modes. From the crossing of an oblique shock to the inner sheath we observe AIC modes propagating toward the magnetopause. Along the Sun-Earth line, from the inner sheath to the magnetopause, AIC modes are observed during one crossing while mirror modes are seen during another crossing. In the sheath depletion layer we observe right-hand polarized AIC modes. At 0600 LT and at equal distance from the magnetopause and from the shock both types of modes are observed, the mirror mode appearing when the magnetic field is draping the magnetopause. From these analysis we infer the source of the propagating AIC modes to be the magnetosheath frontier regions, while the mirror modes grow in situ in the inner magnetosheath when specific conditions are reached.

On the Average Position of the Near-Earth Neutral Sheet.

Magnetic field data from magnetometers on board the ISEE 1 spacecraft are used to examine the position of the magnetotail neutral sheet, which varies in response to variations in the tilt angle of the earth's dipole axis. In the whole data set of the magnetic field from January 1978 to December 1980, 129 neutral sheet crossings are identified in the tail region X<−10 R E |Y|<18 R E and R<22.6 R E . While it is confirmed that the average neutral sheet does warp when the tilt angle is not equal to zero, in contrast to previous warped-neutral sheet models, the points at which the neutral sheet crosses the XY plane move farther away from the sun-earth line, as the absolute value of the tilt angle increases

A 10 yr analysis on the planetary scale of the virtual height of the f-layer

The trend is investigated for 10 yr (1967–1976) of a 24-variate planetary ionospheric index that is derived from the monthly medians of the virtual height of the F-layer ( h′ F) simultaneously recorded within an array of 137 ionospheric observatories. The ionosphere appears to vary generally smoothly, with regular oscillations superimposed over a solar cycle trend, except that, on isolated occasions, when some anomalous features occur within the plane perpendicular to the Sun-Earth line, that is, either to the ‘bulge’ that is observed in the equatorial belt at ~ 18 LT, or to either one of the two ‘cupolas’ observed over the polar caps. Such events ought to be investigated as isolated case histories, apparently uncorrelated with the quasi-periodical smooth oscillations of the normal condition. The periodicities of the regular oscillations include a strong seasonal component, plus periods at ~38, ~44, ~20 and ~ 31–34 months. The strong seasonal variation envisages that h′ F is more sensitive to thermodynamic than to electrodynamic phenomena (unlike geomagnetic records which show a much weaker yearly variation, owing to the large disturbances of the background, thus indicating mostly electrodynamic control). The physical meaning of all other periodicities cannot be suitably investigated by means of the present analysis owing to the possible aliasing caused by tidal oscillations. This cannot be studied when dealing only with monthly median values: for such a purpose, the same analysis ought to be carried out directly using the raw hourly records.

Solar Sail Halo Orbits Part II - Geocentric Case

The equations of motion of a geocentric orbiting high-performance solar sail are analyzed in a corotating reference frame with the axis of corotation directed along the Sun-Earth line. Stationary solutions are found to the equations of motion in the corotating frame that correspond to Earth-centered halo-type orbits when viewed from an inertial frame. The sail performance requirements are minimized and families of linearly stable trajectories identified. By patching these halo orbits together, it will be shown that complex new trajectories may be formed. Geocentric halo orbits may have useful space science and technology applications.

MHD flow past an obstacle: Large‐scale flow in the magnetosheath

As a step to the study of the large‐scale flow in the magnetosheath, the MHD flow past an obstacle is investigated. A time asymptotic method is used to obtain 3D steady‐state solutions. The results indicate the formation of a depletion layer near the obstacle due to the increase of the magnetic field. Along the earth‐sun line the plasma density increases first and then decreases from the post bow shock to the magnetopause. The local density maximum in front of the magnetopause may correspond to what was recently observed. When the interplanetary magnetic field direction is tilted from the solar wind flow, the IMF influences the shape of the bow shock as well as the location of the stagnation point at the magnetopause in a way consistent with observation. In addition, the results show the existence of a magnetosheath current in the post parallel‐shock region.

Solar wind control of the magnetopause shape, location, and motion

We have assembled a data set of 1821 magnetopause crossings. Separate fits to subsets of this data set determine the magnetopause location as a function of solar wind dynamic pressure and interplanetary magnetic field orientation. Solar wind dynamic pressure variations produce self‐similar magnetopause motion on time scales of one hour or longer. We verify the pressure balance relationship between the solar wind dynamic pressure and the location of the subsolar magnetopause. We quantify the relationship between the IMF Bz, region l Birkeland current strength, the position of the subsolar magnetopause, and the shape of the dayside magnetosphere. Cross sections of the dayside magnetopause in planes perpendicular to the Earth‐Sun line are oblate.

On the high-speed plasma streams, stormtime sudden commencements and cosmic-ray intensity: Relation amongst them during epoch 1978 to 1982

We have examined the list of HSPS given by Lindbladt, Lundstedt, and Larsen (1989). The list of SSC and the cosmic-ray intensity registered at Calgary for the same epoch, 1978 to 1982, has been studied and their time variations, using the well-known Chree method of superposed epoch analysis for various groups of data have been carried out. We have shown results that indicate that SSC is an essential pre-requisite in initiating a cosmic-ray Forbush decrease, whereas the occurrence of HSPS alone does not produce a cosmic-ray decrease. Furthermore, our results also clearly reveal that SSC flares originating in a region to the east of the Sun-Earth line reveal a much larger cosmic-ray decrease than SSC associated with flares originating in a region to the west of the Sun-Earth line. Our results substantiate the study of Sarris, Dodopoulos, and Venkatesan (1989), determined for IMP 7 and 8 data (E≥ 35 MeV) for the higher neutron monitor energies. Sarris, Dodopoulos, and Venkatesan have observed that a cosmic-ray intensity decrease follows the group of SSC with flares on the east side of the Sun-spacecraft line, while the group with flares on the west side of the Sun-spacecraft line reveals little or no decrease in cosmic-ray intensity. In particular, the zero epochs for the two groups depend on the hours of onset of SSC. The group with flares on the east side (which are essentially associated with quasi-perpendicular shocks) are followed by cosmic-ray decreases while the group associated with flares on the west of the Sun-spacecraft line (which are usually associated with quasi-parallel shocks) do not reveal any appreciable decrease in cosmic-ray intensity.

Cometary Apparitions: 1990 – 2010

Osculating orbital elements are listed (chronologically by perihelion passage time) for all periodic comets expected to arrive at perihelion during the 1990-2010 interval. Plots which make it possible to readily determine the earth-based viewing conditions of a particular cometary apparition are presented. These plots are drawn in a rotating coordinate system so that the sun-earth line is fixed for each apparition.

Coronal observations with SOHO

Abstract The Solar and Heliospheric Observatory (SOHO) will carry a set of solar physics experiments which permit a thorough investigation of the solar corona. The emphasis of the mission is on the measurement of the physical properties of coronal structures and the processes occurring therein, leading — it is hoped — to an understanding of the mechanism(s) by which the solar corona is heated and the solar wind is accelerated. The observations will be made from a halo orbit around the L1 Lagrange point on the Earth-Sun line and will range from magnetic field measurements in the photosphere through spectroscopic plasma diagnostics of chromospheric, transition-zone and coronal structures with high spatial and spectral resolution, to coronagraphic observations out to 30 solar radii and to mass spectrometry of the solar wind near one astronomical unit. SOHO is part of the Solar Terrestrial Science Programme (STSP), the first ‘Cornerstone’ in ESA's long-term scientific programme ‘Space Science — Horizon 2000’, and will therefore be flown in connection with the other STSP component, the four-spacecraft Cluster mission, which will investigate plasma structures and processes in the magnetosphere in three dimensions. Both STSP missions, i.e. SOHO and Cluster will address the physics of plasma structures and processes, that are accessible to investigation in the solar-terrestrial context, yet are thought to be examples of plasma processes and structures that are ubiquitous in the cosmos. It is hoped that a cross-fertilisation between the scientific communities associated with SOHO and Cluster will take place, as they investigate the physics of the coronal and magnetospheric plasma with complementary methods and techniques — globally by remote observations, and in detail, by multi-point in-situ measurements.

Role of the lifetime of ring current particles on the solar wind-magnetosphere power transfer during the intense geomagnetic storm of 28 August 1978

For the intense magnetic storm of 28 August 1978 it is shown that the power transfer from the solar wind to the magnetosphere is well represented by the expression obtained by Vasyliunas et al. (1982, Planet. Space Sci. 30, 359) from dimensional analysis, but this representation becomes improved when such an expression is modified by a factor due to an influence of the lifetime of ring current particles as suggested by Lee and Akasofu (1984, Planet. Space Sci. 32, 1423). During a steady state regime of the ring current evolution of this storm, our study suggests that the power transfer depends on the solar wind density, the transverse component of the IMF (with respect to the Sun-Earth line) and also, explicitly, on the time consant for ring current energy decay, but not on the solar wind speed.

The deep planetary magnetotail revisited

We extend a previously presented magnetotail model [Grzedzielski and Macek, 1988] to great distances in the antisolar direction. For typical solar wind parameters at ∼ 1 AU and the most probable set of parameters of the model as determined for the ISEE 3 region of ∼ 200 Earth radii (RE), the open geotail extends to at least (3 − 4) × 10³ RE downstream from Earth where it forms a cavity filled with a dense (∼ 1 cm−3), hot ( ≲ 106K) plasma and low magnetic field strengths (1.2 − 0.4 nT). The cross section of this cavity in the plane perpendicular to the Earth‐Sun line has dimensions of 300 − 400 RE parallel to the ecliptic plane, but only ∼ 5 RE in the direction normal to the ecliptic. It seems likely that the magnetotail would become filamentary at such distances. Our results indicate that the Phobos spacecraft could have observed tail associated phenomena at distances of ∼ 3900 RE downstream from Earth during a September 1988 transit. We compare model predictions for the characteristics of planetary magnetotails (including Neptune) in our solar system.