Pradip Gangopadhyay

Heliospheric hydrogen beyond 15 AU: Evidence for a termination shock

The Voyager and Pioneer 10 spacecraft are moving upstream and downstream into the local interstellar flow, monitoring H Lyman α radiation resonantly scattered from heliospheric hydrogen. Voyager Cruise Maneuver observations obtained between 15 and 35 AU reveal that H Lyman α intensities in the upstream direction fall as r−0.75±0.05. Beyond 15 AU downstream, Pioneer 10 intensities fall as r−1.07±0.1. These trends cannot be simultaneously reproduced using a hot H distribution model that does not include termination shock structure. Radiative transfer calculations using the hot H model predict that upstream intensities should fall more rapidly as a function of heliocentric distance than downstream intensities, precisely opposite to the observed trends. The Voyager H Lyman α intensities also show a distinctive trend to decrease less rapidly with increasing heliocentric distance. Between 15 and 20 AU, Voyager intensities fall as r−1, whereas between 30 and 35 AU they fall as r−0.35. This flattening trend implies that the upstream H density is increasing rapidly with heliocentric distance beyond ≈25 AU. A simple analysis suggests that the density distribution changes from nearly uniform between 15 and 20 AU, to r0.65 dependence between 30 and 35 AU. This steepening trend is significant because similar H density gradients are predicted in models which include the effects of the termination shock. Taken together, the Voyager and Pioneer 10 H Lyman α observations beyond 15 AU imply the existence of a solar wind termination shock, suggesting that it lies between 70 and 105 AU in the upstream direction.

First‐year continuous solar EUV irradiance from SOHO by the CELIAS/SEM during 1996 solar minimum

The CELIAS/SEM photodiode spectrometer aboard SOHO continuously monitors the full-disk EUV solar irradiance in an 8-nm wavelength band centered at 30.4 nm (first order), and in a broad wavelength band between 0.1 and 77 nm (central order). We present the absolute solar EUV irradiances for the 1996 solar EUV minimum year at 1 AU. The uncertainty in absolute flux for each channel is approximately ± 14%. The accuracy and stability of the instrument make the data extremely useful in modeling the upper terrestrial atmosphere during this period of “low” solar activity. The data show evidence of persistent solar EUV/soft X ray active regions throughout this solar minimum period which give rise to both 27-day and short-term (minutes to hours) solar EUV irradiance variations. The lowest value of solar flux in the first order 30.4-nm band occurred on November 6, 1996, with a photon flux of 9.8 × 109 cm−2 s−1. Using previously obtained solar spectra, we infer a photon flux of 4.7 × 109 cm−2 s−1 within a 1-nm bandpass centered on the solar He II 30.4-nm emission line at this time. The irradiance variation of the first order channel was between +15% and −10% as measured from a smoothed quadratic least squares fit to the entire first-order channel database for 1996. The lowest central-order EUV photon flux occurred on the same day (November 6, 1996) with an absolute flux of 2.2 × 1010 cm−2 s−1. When sharp increases of short-term flux variability are ignored, a variation between +45% and −30% from the smoothed least squares fit to the central-order database is obtained. The long-term solar cycle valuation during the 12-month smoothed data in both channels indicates that the solar EUV minimum was reached during mid 1996. Large short-term sudden increases monitored by both channels correspond to solar flares observed from the ground and from the GOES satellites. New data for two isolated flares obtained from both CELIAS/SEM channels are also presented and compared with GOES 0.1 to 0.8-nm soft X ray data.

The SOHO CELIAS/SEM EUV database from SC23 minimum to the present

Abstract The SOHO Solar EUV Monitor has been in operation since December 1995 onboard the SOHO spacecraft. This instrument is a highly stable transmission grating solar extreme ultraviolet spectrometer. It has made nearly continuous full disk solar irradiance measurements both within an 8 nm bandpass centered at 30.4 nm and throughout the 0.1 to 50 nm solar flux region since launch. The 30.4 nm flux, the 0.1 to 50 nm flux and the extracted soft X-ray (0.1 to 5 nm) flux are presented and compared with the behavior of solar proxies.

Evidence of a nearby solar wind shock as obtained from distant Pioneer 10 ultraviolet glow data

Pioneer 10 UV photometric data are used to study the nature of the heliospheric boundary interactions between solar wind and inflowing partially ionized interstellar wind and the interstellar magnetic field. The radial intensity dependence of solar Ly-alpha photons backscattered by inflowing interstellar hydrogen atoms is observed. The data in the downstream direction at distances greater than about 37 AU show a distortion of the incoming hydrogen atom density profile. It is suggested that this is characteristic of a nearby solar wind shock probably located about 50 AU from the sun in January 1987. 58 references.

The hydrogen density of the local interstellar medium and an upper limit to the Galactic glow determined from Pioneer 10 ultraviolet photometer observations

Pioneer 10 Lyman-alpha data obtained beyond the heliocentric distance of 30 AU are used here to determine the local ISM hydrogen density and downwind Galactic glow. The resulting asymptotic interstellar hydrogen density obtained using a conventional interplanetary model is 0.06/cu cm, and the Galactic Lyman-alpha glow is found to be negligible in the downwind direction. Discrepancies are found between observations and the conventional theoretical predictions of the glow dependence on radial distance. Based solely on data obtained between 30 and 39 AU, a better fit is obtained with a constant density distribution beyond 30 AU. In this case, a density of 0.05/cu cm and Galactic glow of 1.5 R is obtained. These results suggest that a complex density distribution may be more appropriate than either the conventional exponential distribution or the constant distribution. 43 references.

Voyager 1, Voyager 2, and Pioneer 10 Lyα data and their interpretation

The Voyager 1 (V1), Voyager 2 (V2), and Pioneer 10 (P10) Lyα data sets are three of several diagnostic data sets available for the study of the very local interstellar medium (VLISM). Selected V1 data obtained on 1989 day 279 at heliocentric distance of 39.1 AU in the upstream direction relative to the incoming interstellar neutral hydrogen flow and V2 data obtained on 1990 day 143 at heliocentric distance of 32 AU, also in the upstream direction, have been used to estimate the local interstellar neutral hydrogen and proton densities and compared with P10 data obtained in 1981 at distances between 23.39 and 23.87 AU in the downstream direction, respectively. State-of-the-art plasma-neutral and radiative transfer models have been used in the interpretation of the data. It has been found that a VLISM heliospheric model with neutral hydrogen density of 0.18 cm-3 and proton density of 0.06 cm-3 best fits both the V1 data and the V2 data. The P10 data are best fitted by a VLISM model with neutral hydrogen density of 0.15 cm-3 and proton density of 0.05 cm-3. The failure to find a single best-fit stationary heliosphere plasma-neutral model suggests, among other possibilities, that a quantitative interpretation of the heliospheric Lyα glow would require the incorporation of magnetic field and time dependence in the heliospheric model.

Reappraisal of the Pioneer 10 and Voyager 2 Lyα intensity measurements

The Pioneer 10 (P10) and Voyager 2 (V2) calibration difference of 4.4 at Lyα has made it difficult to interpret the Lyα data and also to resolve the outer planetary upper atmosphere excess Lyα glow problem. We have carried out radiative transfer calculations using an improved radiative transfer code and six heliosphere neutral-plasma density models to study the calibration of P10 and V2 at Lyα and found that both P10 and V2 intensity measurements are in need of revision. The intercalibration difference is discussed using our model calculations, recent large-distance neutral hydrogen density determinations obtained from pickup-ion and solar wind slow-down data, the recent change in the estimate of the solar Lyα flux values, and Voyager 1 energetic particle measurements. These recent heliospheric measurements and Lyα glow model calculations support the need for an upward revision of P10 and a downward revision of V2 Lyα intensity. It is not yet possible to give a definitive estimate of the required revision because of lack of knowledge of the very local interstellar medium neutral hydrogen density. The calibration revision is found to reduce the range of variation of Jovian dayglow.

SEP Temporal Fluctuations Related to Extreme Solar Flare Events Detected by SOHO/CELIAS/SEM

The SOHO CELIAS/SEM measurements of the solar Extreme Ultraviolet (EUV) irradiance in the He II 30.4 nm first-order channel (26–34 nm) are highly sensitive to impacts of Solar Energetic Particles (SEP). A model of the SEM response to a quasi-isotropic SEP fluence allowed us to determine both the range of proton incident energies and the SEM sensitivity to the SEP flux, which has a maximum around 12 MeV. We propose to use high-cadence (15 s) SEM first-order count-rates to analyze the temporal fluctuations of the SEP flux arriving from extreme solar flare events. A comparison of these temporal fluctuations for the July 14, 2000, October 28, November 2, November 4, 2003, and January 20, 2005 events shows that the most intense high-frequency RMS variations of the first-order count-rates at the time when SEPs started to arrive, were associated with the January 20 event. These high-frequency variations are produced by packets of SEPs. Two (plus and minus) first-order SEM detectors with the distance between them of about 62 mm allow us to analyze the spatial coincidences and temporal distribution of SEP related signals at 1 AU. The largest temporal separation of the SEP packets is observed at the time when the packets start to arrive. RMS fluctuations (variances) for all analyzed events do not follow the photon noise ( √ N) in assumption of the Poisson or normal distribution but correlate (R = 0.999) with the speed at which the SEP flux grows. If the found correlation is confirmed on a larger statistical data base, it may allow the prediction of the SEP flux growth profile by analyzing the RMS amplitudes for the initial phase of the SEP impact.

The SoHO CELIAS/SEM data base

Abstract The CELIAS/SEM spectrometer aboard SoHO continuously monitors the full disk solar EUV and XUV irradiance. The instrumental noise level of the SEM is less than 0.5% of the signal in all channels, hence the short term signal variations larger than this in the SEM data base represent true solar variations in the EUV/XUV flux. Thrree sounding rocket calibration underflights since the SoHO launch have provided absolute EUV/soft X-ray solar flux values with which the long term stability of the SoHO SEM can be determined. In each of those flights the SEM “clone” has been calibrated before and after launch at the SURF II and SURF III synchrotron radiation facility of NIST in Gaithersburg, Maryland. The relative uncertainty of the SEM clone over a three year period is only ± 5.5% (1σ) and yields an absolute uncertainty of only 8.5% in transfering the absolute flux calibration to the SoHO CELIAS/SEM instrument.

A study of the effect and consequence of solar cycle variation on low-energy galactic cosmic rays using Pioneer 10 H Lyman α data as a solar cycle proxy

The correlation of the 1981–1990 (∼ 23 to ∼ 50 AU) data of three instruments, the ultraviolet photometer (UV), the charged particle instrument (CPI), and the trapped radiation detector (TRD), on board Pioneer 10 is investigated. The reduction of the data necessary for applying a fast Fourier transform (FFT) algorithm is described. A cross correlation function between the hydrogen Lyman a backscattered radiation, used as a solar cycle proxy, and the cosmic ray fluxes is obtained from the FFT algorithm. The time lag in the anticorrelation function between the galactic cosmic rays and the Lyman α line is used to determine the distance to the solar wind termination shock (75 AU), under the assumption that the cosmic ray modulation boundary for low-energy particles and the termination shock are coincident. The ratio of the upwind termination shock distance to the downwind shock distance is determined to be 2/3 using the Voyager 1 position in 1996.