IAGA Magnetic Repeat Station Survey – Condensed Notes

Key Terminology

• Repeat station: fixed ground mark where absolute vector observations are made at a standard height.
• Normal field: main + static crustal field after removing external contributions.
• DIM: Declination–Inclination magnetometer (fluxgate-theodolite).
• PPM: Proton precession (or Overhauser) magnetometer for $F$.
• Variometer: 3-axis fluxgate (records $X,Y,Z$ or $D,H,Z$ continuously).
• Survey classes
  • V – local variometer • M – reference observatory • A – absolutes only
  • 1 = full correction; 2 = approx. correction; 3 = spot values.

Accuracy Targets

• Observatory standard: $\le1\,\text{nT}$ (intensity), $\le0.1'$ (direction).
• Practical repeat-station goal: $\approx5\,\text{nT}$ or $1'$ $D$, $0.5'$ $I$.
• Crucial to correct transient fields; more valuable to have few high-quality stations than many poor ones.

Network Design

• Spacing ≈$200\,\text{km}$ (core SV resolves >$400\,\text{km}$).
• Reoccupation interval:
  • Charts only → 5 yr; • SV modelling → 2 yr; • Jerks/high SV → 1 yr.
• Use primary + secondary marks (≥200 m apart) to guard against loss/contamination.

Site Selection

• Low field gradients (<50 nT / 10 m).
• Away from magnetic noise (DC rail, power, transmitters).
• Secure, accessible, long-term permanence.
• Provide ≥4 azimuth marks; verify each visit.

Station Construction

• Pillar or ground slab with tripod sockets; record standard instrument height.
• Mark auxiliary $F$ point 5–10 m away.
• Document: coordinates, photos, gradient map, reference-mark diagram.

Instrument Suite

• DIM on non-magnetic theodolite (Zeiss 020B/010B). Align fluxgate; verify offset.
• PPM: set frequencies to $26751.525581\times10^{4}\,\text{rad T}^{-1}\text{s}^{-1}$; compare to standard; keep sensor orientation fixed.
• 3-axis variometer: insulate/bury sensor; log 1-min means; determine scale & $\text{d}E/\text{d}T$.

Field Procedure (1st-order, on-site variometer)

1. Install variometer, level, orient, start clock.
2. Verify station & gradients; check reference-mark angles.
3. Determine azimuth: preferred Sun hour-angle method; gyro or differential GPS if cloudy.
4. Absolute sets (typ. $FDI|I|DF$) early a.m. & late p.m.; aim for night runs; simultaneous $F$ at auxiliary.
5. Occupy ≥2 days to tie baselines; repeat on secondary mark for check.

Data Reduction

Using Reference Observatory

$E = E<em>o + \bigl[ E(t)-E</em>o(t) \bigr] + (SV-SV_o)\,\Delta T$

• Assumes identical transients; best when station–obs < few 100 km and mid-year occupation.

Using Local Variometer

• Derive baselines from absolutes; correct minute data → hourly → night quiet means.
• Apply observatory long-term record to offset residual night displacement.
• Compensate temperature drift: fit $E<em>{bl}=a+b(T-T</em>0)$.

Error Sources

• Instrument: DIM $20''$, $\pm0.5$ nT PPM.
• Mis-centering: error $=\nabla F \times \delta r$; control to cm level.
• Observatory mismatch; crustal induction (can vary over <10 km).
• Non-uniform SV when reducing to annual mean.

Special Regions

• Equatorial electrojet (±3° dip): daytime $H$ spikes >$100$ nT; use stations at same geomag-lat or local variometer; observe $<09$ LT & >18 LT.
• Auroral zone (geomag lat $60–73^{\circ}$): frequent night storms; occupy ~1 week; late-summer best; observatory correction unreliable.
• Polar cap (>$77^{\circ}$): summer IMF offsets in $Z$ up to $60$ nT; autumn–winter surveys; observatory interpolation often OK.

Reporting (IAGA scheme)

• Submit for each occupation: Regional network description, Record sheet, Computer file.
• Classification code: e.g. $V1.0$ (local vario, full correction).
• Send to WDC-A (Boulder) & IAGA WG V-8.

Minimal Checklist Before Leaving Site

• Gradient re-measured; photos taken.
• Absolute sets concur (≤$1'$, 5 nT).
• Variometer clock synced; data backed-up.
• Station description & status report updated.
• Local authorities thanked.